docs(website): fill the six Reference stubs with worked examples + output
Expand columns, relationships, indexes, constraints, inserting-and-editing- data, and querying-and-inspecting from syntax-only stubs into full pages, each with worked examples on the library schema and real captured app output (structure boxes, relationship diagrams, data tables, show-lists, query plans, cascade summaries). All output captured verbatim from the app — never hand-drawn. Both simple- and advanced-mode forms shown where both apply; advanced syntax verified against tests/source. STYLE.md: record the output-block convention (plain unlabelled fence, captured from a throwaway harness, not hand-drawn). Verified: pnpm build clean (24 pages); no forbidden terms; internal links and heading anchors resolve.
This commit is contained in:
claude@clouddev1
@@ -5,12 +5,92 @@ sidebar:
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order: 3
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---
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:::note[In progress]
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The syntax below is complete and accurate; worked examples on the library
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schema are being added.
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:::
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A table's columns are not fixed once it exists. You can add new ones, rename
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them, change their type, and remove them — in both simple and advanced mode.
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The examples below use the [example library](/getting-started/example-library/);
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to see a table's current columns at any time, use `show table`:
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After a table exists, you add and change its columns. In simple mode:
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```rdbms
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show table books
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```
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```
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books
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┌───────────┬────────┬─────────────┐
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│ Name │ Type │ Constraints │
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├───────────┼────────┼─────────────┤
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│ book_id │ serial │ PK │
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│ title │ text │ NOT NULL │
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│ author_id │ int │ │
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│ published │ int │ │
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│ isbn │ text │ UNIQUE │
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└───────────┴────────┴─────────────┘
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```
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## Add a column
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```rdbms
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add column to books: page_count (int)
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```
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The new column goes at the end of the table and is empty for existing rows.
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After the change, the updated structure is shown automatically. If you add a
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`serial` or `shortid` column — or change a column *to* one of those types —
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the empty cells are filled with freshly generated values in the same step (see
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[Types](/reference/types/)).
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## Rename a column
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```rdbms
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rename column in books: page_count to pages
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```
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## Change a column's type
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```rdbms
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change column in books: pages (text)
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```
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Changing a type re-interprets every existing value. When the conversion could
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lose information — turning `text` into `int`, say, where some rows do not look
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like numbers — the playground refuses by default and shows you exactly which
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rows are the problem, so nothing is silently corrupted. Two flags let you
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decide what should happen instead:
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- `--force-conversion` — go ahead and convert, accepting the loss.
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- `--dont-convert` — keep the column's stored values as they are under the new
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type.
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A primary-key column, or a column taking part in a
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[relationship](/reference/relationships/), is protected — remove the
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relationship first if you need to restructure it.
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## Drop a column
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```rdbms
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drop column from books: pages
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```
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If the column is covered by an [index](/reference/indexes/), the drop is
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refused so you do not lose the index by surprise. Add `--cascade` to drop the
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column and the index that covers it together:
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```rdbms
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drop column from books: pages --cascade
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```
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## In advanced mode
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The same operations are written as standard `ALTER TABLE` statements:
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```sql
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alter table books add column page_count int
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alter table books rename column page_count to pages
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alter table books alter column pages type text
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alter table books drop column pages
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```
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## Syntax
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```rdbms-syntax
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add column [to] [table] <Table>: <Name> (<Type>)
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@@ -19,11 +99,5 @@ change column [in] [table] <Table>: <Name> (<Type>) [--force-conversion | --dont
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drop column [from] [table] <Table>: <Name> [--cascade]
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```
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Changing a column to `serial` or `shortid` fills existing rows with generated
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values. A primary-key column, or one involved in a relationship, is refused
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until you remove the relationship first.
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In advanced mode the same operations are `ALTER TABLE … ADD/DROP/RENAME
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COLUMN` and `ALTER TABLE … ALTER COLUMN … TYPE`.
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See also [Types](/reference/types/) and [Constraints](/reference/constraints/).
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See also [Types](/reference/types/), [Constraints](/reference/constraints/),
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and [Tables](/reference/tables/).
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@@ -1,17 +1,99 @@
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---
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title: Constraints
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description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints on columns.
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description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints that keep data valid.
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sidebar:
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order: 6
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---
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|
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:::note[In progress]
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The syntax below is complete and accurate; worked examples on the library
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schema are being added.
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:::
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Constraints are rules the database enforces on a column's values, so invalid
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data can never be stored in the first place. There are four:
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Constraints keep your data valid. Declare them in the column spec at
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`create table` / `add column` time, or change them on an existing column:
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| Constraint | Rule |
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|---|---|
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| `not null` | The column must always have a value. |
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| `unique` | No two rows may share a value (empty values aside). |
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| `default <value>` | The value to use when an `insert` omits the column. |
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| `check (<expr>)` | Every value must satisfy a condition you write. |
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## Adding constraints
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You can add a constraint to an existing column at any time. In the
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[example library](/getting-started/example-library/), a book's publication year
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should be positive, and a sensible fallback is useful when it is unknown:
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```rdbms
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add constraint default 2000 to books.published
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add constraint check (published > 1400) to books.published
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```
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Constraints show up in the column's **Constraints** cell in `show table`:
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```rdbms
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show table books
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```
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```
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books
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┌───────────┬────────┬──────────────────────────────────────────┐
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│ Name │ Type │ Constraints │
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├───────────┼────────┼──────────────────────────────────────────┤
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│ book_id │ serial │ PK │
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│ title │ text │ NOT NULL │
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│ author_id │ int │ │
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│ published │ int │ DEFAULT 2000, CHECK ("published" > 1400) │
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│ isbn │ text │ UNIQUE │
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└───────────┴────────┴──────────────────────────────────────────┘
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```
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Here `title` is `NOT NULL` and `isbn` is `UNIQUE` — both were declared when the
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table was built (see below). `PK` marks the primary key, which is fixed at
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creation time ([Tables](/reference/tables/)).
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## Declaring constraints when you build the table
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Constraints can also be part of a column's spec from the start. In advanced
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mode they sit inline in `CREATE TABLE`:
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```sql
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create table books (
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book_id serial primary key,
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title text not null,
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author_id int,
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published int default 2000 check (published > 1400),
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isbn text unique
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)
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```
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## Adding a constraint to existing data
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When you add a constraint to a column that already holds rows, the playground
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checks the current data first. If any row would break the new rule — a `null`
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where you are adding `not null`, a duplicate where you are adding `unique`, or a
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value that fails a `check` — the change is **refused** and the offending rows
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are listed, so you can fix the data and try again. Nothing is changed until the
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constraint can hold for every row.
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## Removing a constraint
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Name the kind to drop it (there is at most one of each kind per column):
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```rdbms
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drop constraint check from books.published
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drop constraint default from books.published
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```
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## In advanced mode
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A table-level `check` or `unique` can be added and dropped with `ALTER TABLE`;
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a named constraint can then be dropped by that name:
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```sql
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alter table books add constraint published_range check (published > 1400)
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alter table books add unique (title, author_id)
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alter table books drop constraint published_range
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```
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## Syntax
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```rdbms-syntax
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add constraint not null to <Table>.<col>
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@@ -21,10 +103,5 @@ add constraint check (<expr>) to <Table>.<col>
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drop constraint (not null | unique | default | check) from <Table>.<col>
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```
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Adding a constraint to a column that already has rows is checked first — if
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existing data would violate it, the change is refused and the offending rows
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are shown.
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In advanced mode these are the standard column- and table-level constraints
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in `CREATE TABLE` / `ALTER TABLE`. The primary key is fixed at creation (see
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[Tables](/reference/tables/)).
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See also [Columns](/reference/columns/), [Types](/reference/types/), and
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[Inserting & editing data](/reference/inserting-and-editing-data/).
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@@ -1,16 +1,92 @@
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---
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title: Indexes
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description: Create and drop indexes to speed up lookups.
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description: Create and drop indexes to speed up lookups, and see their effect with query plans.
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sidebar:
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order: 5
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---
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|
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:::note[In progress]
|
||||
The syntax below is complete and accurate; worked examples on the library
|
||||
schema are being added.
|
||||
:::
|
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An index is a lookup structure that lets the database find matching rows
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without scanning the whole table. Looking up a book by its author is a common
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query in the [example library](/getting-started/example-library/), so an index
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on `books.author_id` is worthwhile:
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|
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An index speeds up lookups on one or more columns. In simple mode:
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```rdbms
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add index as idx_books_author on books (author_id)
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```
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`show indexes` lists every index in the project, qualified by its table:
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```rdbms
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show indexes
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```
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```
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Indexes (1):
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books.idx_books_author (author_id)
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```
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## Seeing the difference an index makes
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Prefix a query with `explain` to see *how* the database would run it, without
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running it (see [Query plans](/reference/querying-and-inspecting/#query-plans)).
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With the index in place, a lookup on `author_id` uses it:
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```rdbms
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explain show data books where author_id = 1
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```
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```
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SELECT "book_id", "title", "author_id", "published", "isbn" FROM "books" WHERE "author_id" = 1
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└─ SEARCH books USING INDEX idx_books_author (author_id=?)
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```
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A filter on a column with no index has to read every row instead — a **scan**:
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```rdbms
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explain show data books where published > 1968
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```
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```
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SELECT "book_id", "title", "author_id", "published", "isbn" FROM "books" WHERE "published" > 1968
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└─ SCAN books
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```
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`SEARCH … USING INDEX` is the fast path; `SCAN` reads the whole table. On a few
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rows the difference is invisible, but it is the core idea behind why indexes
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exist — and the plan tree lets you see which one your query gets.
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## Multi-column indexes
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An index can cover more than one column; list them in order. This helps
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queries that filter or sort on that same leading combination:
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|
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```rdbms
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add index as idx_loans_book_member on loans (book_id, member_id)
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```
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## Dropping an index
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Drop it by name, or by the columns it covers:
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|
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```rdbms
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drop index idx_books_author
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drop index on books (author_id)
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```
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## In advanced mode
|
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```sql
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create index idx_books_author on books (author_id)
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create unique index idx_books_isbn on books (isbn)
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drop index idx_books_author
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```
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|
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A `unique` index doubles as a uniqueness constraint — it both speeds up
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lookups and rejects duplicate values. (In simple mode, uniqueness is a
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[constraint](/reference/constraints/) on the column rather than an index
|
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option.)
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|
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## Syntax
|
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```rdbms-syntax
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add index [as <Name>] on <Table> (<col>[, ...])
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@@ -18,8 +94,5 @@ drop index <Name>
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drop index on <Table> (<col>[, ...])
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```
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|
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In advanced mode, `CREATE [UNIQUE] INDEX … ON <Table> (…)` and `DROP INDEX`.
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Unique indexes are available on the advanced surface.
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|
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To see how an index changes the way a query runs, use
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[Query plans](/reference/querying-and-inspecting/).
|
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See also [Constraints](/reference/constraints/) and
|
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[Querying & inspecting](/reference/querying-and-inspecting/).
|
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|
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@@ -1,26 +1,117 @@
|
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---
|
||||
title: Inserting & editing data
|
||||
description: Add, change, and remove rows.
|
||||
description: Add, change, and remove rows — with required filters and automatic confirmation of what changed.
|
||||
sidebar:
|
||||
order: 7
|
||||
---
|
||||
|
||||
:::note[In progress]
|
||||
The syntax below is complete and accurate; worked examples on the library
|
||||
schema are being added.
|
||||
:::
|
||||
Once your tables exist, you fill them with rows and keep them up to date. The
|
||||
examples use the [example library](/getting-started/example-library/). After
|
||||
every write, the playground shows you the rows that changed, so you can confirm
|
||||
the effect at a glance.
|
||||
|
||||
## Inserting rows
|
||||
|
||||
List the columns you are providing, then the matching values:
|
||||
|
||||
```rdbms
|
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insert into members (name, joined) values ('Katherine Johnson', '2024-06-01')
|
||||
```
|
||||
|
||||
```
|
||||
1 row(s) inserted
|
||||
┌───────────┬───────────────────┬────────────┐
|
||||
│ member_id │ name │ joined │
|
||||
├───────────┼───────────────────┼────────────┤
|
||||
│ 3 │ Katherine Johnson │ 2024-06-01 │
|
||||
└───────────┴───────────────────┴────────────┘
|
||||
```
|
||||
|
||||
Notice that `member_id` was filled in automatically — it is a `serial` column,
|
||||
so you leave it out of the `insert` and the database assigns the next value.
|
||||
The same applies to `shortid` columns and to any column with a
|
||||
[default](/reference/constraints/) (see [Types](/reference/types/)).
|
||||
|
||||
## Updating rows
|
||||
|
||||
`update` changes columns on the rows that match a filter:
|
||||
|
||||
```rdbms
|
||||
update books set published = 1970 where book_id = 2
|
||||
```
|
||||
|
||||
```
|
||||
1 row(s) updated
|
||||
┌─────────┬───────────────────────────┬───────────┬───────────┬────────────────┐
|
||||
│ book_id │ title │ author_id │ published │ isbn │
|
||||
├─────────┼───────────────────────────┼───────────┼───────────┼────────────────┤
|
||||
│ 2 │ The Left Hand of Darkness │ 1 │ 1970 │ 978-0441478125 │
|
||||
└─────────┴───────────────────────────┴───────────┴───────────┴────────────────┘
|
||||
```
|
||||
|
||||
## Deleting rows
|
||||
|
||||
```rdbms
|
||||
delete from authors where author_id = 3
|
||||
```
|
||||
|
||||
When the deleted row has children linked by an `on delete cascade`
|
||||
[relationship](/reference/relationships/), those children are removed too, and
|
||||
the result reports each affected relationship:
|
||||
|
||||
```
|
||||
1 row(s) deleted
|
||||
related: 1 row(s) deleted in `books` for relationship `books_author` (on delete cascade)
|
||||
```
|
||||
|
||||
## The required filter
|
||||
|
||||
`update` and `delete` **must** have a `where` clause. This is a safety rail: it
|
||||
is far too easy to wipe or rewrite a whole table by forgetting one. When you
|
||||
really do mean every row, say so explicitly with `--all-rows`:
|
||||
|
||||
```rdbms
|
||||
update books set published = 2000 --all-rows
|
||||
delete from loans --all-rows
|
||||
```
|
||||
|
||||
```rdbms-syntax
|
||||
update <Table> set <col>=<value>[, ...] (where <expr> | --all-rows)
|
||||
delete from <Table> (where <expr> | --all-rows)
|
||||
```
|
||||
|
||||
## Richer filters
|
||||
|
||||
A `where` clause is more than simple equality. It accepts `and` / `or` /
|
||||
`not`, the comparison operators, and `like`, `is null`, `in`, and `between`:
|
||||
|
||||
```rdbms
|
||||
delete from loans where returned_on is null and loaned_on < '2024-01-01'
|
||||
```
|
||||
|
||||
The same expression grammar drives `show data` — see
|
||||
[Querying & inspecting](/reference/querying-and-inspecting/).
|
||||
|
||||
## In advanced mode
|
||||
|
||||
The simple-mode commands above already use SQL-like syntax; advanced mode is
|
||||
full SQL, including multi-row inserts and `returning`:
|
||||
|
||||
```sql
|
||||
insert into members (name, joined) values
|
||||
('Katherine Johnson', '2024-06-01'),
|
||||
('Dorothy Vaughan', '2024-06-02')
|
||||
update books set published = 1970 where book_id = 2
|
||||
delete from authors where author_id = 3
|
||||
```
|
||||
|
||||
## Syntax
|
||||
|
||||
```rdbms-syntax
|
||||
insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])
|
||||
update <Table> set <col>=<value>[, ...] (where <col>=<value> | --all-rows)
|
||||
delete from <Table> (where <col>=<value> | --all-rows)
|
||||
update <Table> set <col>=<value>[, ...] (where <expr> | --all-rows)
|
||||
delete from <Table> (where <expr> | --all-rows)
|
||||
```
|
||||
|
||||
`update` and `delete` require a `where` clause; to act on every row you must
|
||||
opt in explicitly with `--all-rows`. Auto-generated columns (`serial`,
|
||||
`shortid`) fill themselves in, so you can leave them out of an `insert`. After
|
||||
a write, the affected rows are shown automatically.
|
||||
|
||||
`where` accepts more than simple equality — `and` / `or` / `not`,
|
||||
comparisons, `like`, `is null`, `in`, and `between`. See
|
||||
[Querying & inspecting](/reference/querying-and-inspecting/).
|
||||
See also [Querying & inspecting](/reference/querying-and-inspecting/) and
|
||||
[Constraints](/reference/constraints/).
|
||||
|
||||
@@ -1,43 +1,185 @@
|
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---
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title: Querying & inspecting
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description: View rows and schema, run SQL queries, and explain how a query runs.
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description: View rows and schema, run SQL queries with joins, and explain how a query runs.
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sidebar:
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order: 8
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---
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:::note[In progress]
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The syntax below is complete and accurate; worked examples on the library
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schema are being added.
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:::
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This page covers reading what is in your project: the rows in a table, the
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shape of the schema, full SQL queries, and the plan the database uses to run a
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query. Examples use the [example library](/getting-started/example-library/).
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## Viewing rows and schema (simple mode)
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## Viewing rows
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```rdbms-syntax
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show data <Table> [where <expr>] [limit <n>]
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show table <Table>
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show tables
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show relationships
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show indexes
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show relationship <Name>
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show index <Name>
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`show data` prints a table's rows:
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```rdbms
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show data books
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```
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## Querying (advanced mode)
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```
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┌─────────┬───────────────────────────┬───────────┬───────────┬────────────────┐
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│ book_id │ title │ author_id │ published │ isbn │
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├─────────┼───────────────────────────┼───────────┼───────────┼────────────────┤
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│ 1 │ A Wizard of Earthsea │ 1 │ 1968 │ 978-0553383041 │
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│ 2 │ The Left Hand of Darkness │ 1 │ 1969 │ 978-0441478125 │
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│ 3 │ Invisible Cities │ 2 │ 1972 │ 978-0156453806 │
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│ 4 │ Kindred │ 3 │ 1979 │ 978-0807083697 │
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└─────────┴───────────────────────────┴───────────┴───────────┴────────────────┘
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```
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Advanced mode runs full SQL `select`, including joins, `group by`, `order
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by`, set operations, and `with` (CTEs):
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Add a `where` clause to filter, and `limit` to cap the number of rows:
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```rdbms-syntax
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select (* | <expr> [as <alias>][, ...]) from <Table> [where <expr>] [order by <expr> [asc|desc][, ...]] [limit <n>]
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```rdbms
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show data books where published > 1968
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```
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```
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┌─────────┬───────────────────────────┬───────────┬───────────┬────────────────┐
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│ book_id │ title │ author_id │ published │ isbn │
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├─────────┼───────────────────────────┼───────────┼───────────┼────────────────┤
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│ 2 │ The Left Hand of Darkness │ 1 │ 1969 │ 978-0441478125 │
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│ 3 │ Invisible Cities │ 2 │ 1972 │ 978-0156453806 │
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│ 4 │ Kindred │ 3 │ 1979 │ 978-0807083697 │
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└─────────┴───────────────────────────┴───────────┴───────────┴────────────────┘
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```
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The `where` grammar is the same one used by `update` and `delete` — see
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[Inserting & editing data](/reference/inserting-and-editing-data/#richer-filters).
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## Inspecting the schema
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`show table` describes one table's columns, relationships, and indexes (see
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[Columns](/reference/columns/) and [Relationships](/reference/relationships/)
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for the full output). The plural forms give a project-wide overview:
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```rdbms
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show tables
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```
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```
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Tables (4):
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authors
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books
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loans
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members
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```
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```rdbms
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show relationships
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```
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```
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Relationships (3):
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books_author: authors.author_id → books.author_id on delete cascade
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loans_book: books.book_id → loans.book_id on delete cascade
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loans_member: members.member_id → loans.member_id on delete cascade
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```
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```rdbms
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show indexes
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```
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```
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Indexes (1):
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books.idx_books_author (author_id)
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```
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The singular `show relationship <name>` and `show index <name>` show one item
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in detail — `show relationship` draws the two-table diagram covered in
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[Relationships](/reference/relationships/#viewing-a-relationship).
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## Querying in advanced mode
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Advanced mode runs full SQL `select`, including projections, `order by`,
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joins, `group by`, set operations, and `with` (CTEs). A projection of two
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columns, newest first:
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```sql
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select title, published from books where published > 1968 order by published desc
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```
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```
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┌───────────────────────────┬───────────┐
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│ title │ published │
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├───────────────────────────┼───────────┤
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│ Kindred │ 1979 │
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│ Invisible Cities │ 1972 │
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│ The Left Hand of Darkness │ 1969 │
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└───────────────────────────┴───────────┘
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```
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A **join** follows a relationship to combine rows from two tables — here, each
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book with its author's name:
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```sql
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select authors.name, books.title from books
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join authors on books.author_id = authors.author_id
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order by authors.name
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```
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```
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┌───────────────────┬───────────────────────────┐
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│ name │ title │
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├───────────────────┼───────────────────────────┤
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│ Italo Calvino │ Invisible Cities │
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│ Octavia E. Butler │ Kindred │
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│ Ursula K. Le Guin │ A Wizard of Earthsea │
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│ Ursula K. Le Guin │ The Left Hand of Darkness │
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└───────────────────┴───────────────────────────┘
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```
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Add `group by` with an aggregate to summarise — how many books each author has:
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```sql
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select authors.name, count(*) as book_count from books
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join authors on books.author_id = authors.author_id
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group by authors.name
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order by book_count desc
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```
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```
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┌───────────────────┬────────────┐
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│ name │ book_count │
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├───────────────────┼────────────┤
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│ Ursula K. Le Guin │ 2 │
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│ Octavia E. Butler │ 1 │
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│ Italo Calvino │ 1 │
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└───────────────────┴────────────┘
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```
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## Query plans
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Prefix a query with `explain` to see how the database would run it, rendered
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as an annotated tree — without running it (so it is safe even over `update`
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or `delete`):
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Prefix a query with `explain` to see *how* the database would run it, drawn as
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an annotated tree. `explain` never runs the statement — it only reports the
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plan — so it is safe even over a `delete`:
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```rdbms
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explain show data books where author_id = 1
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```
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```
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SELECT "book_id", "title", "author_id", "published", "isbn" FROM "books" WHERE "author_id" = 1
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└─ SEARCH books USING INDEX idx_books_author (author_id=?)
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```
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`SEARCH … USING INDEX` means the query found its rows through an
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[index](/reference/indexes/) instead of reading the whole table. A filter on an
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un-indexed column shows a `SCAN` instead. In advanced mode, `explain` also
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wraps a `select`, `with`, `insert`, `update`, or `delete`.
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## Syntax
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```rdbms-syntax
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show data <Table> [where <expr>] [limit <n>]
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show table <Table>
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show tables | show relationships | show indexes
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show relationship <Name> | show index <Name>
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select (* | <expr> [as <alias>][, ...]) from <Table> [join …] [where <expr>] [group by …] [order by <expr> [asc|desc]] [limit <n>]
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explain show data <Table> [where <expr>]
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explain <select | with | insert | update | delete …> (advanced mode)
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explain <select | with | insert | update | delete …>
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```
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See also [Indexes](/reference/indexes/),
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[Inserting & editing data](/reference/inserting-and-editing-data/), and the
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[Tables](/reference/tables/) reference.
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@@ -1,17 +1,140 @@
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---
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title: Relationships
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description: Declare and remove foreign-key relationships between tables.
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description: Declare, view, and remove foreign-key relationships between tables.
|
||||
sidebar:
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order: 4
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||||
---
|
||||
|
||||
:::note[In progress]
|
||||
The syntax below is complete and accurate; worked examples on the library
|
||||
schema are being added.
|
||||
:::
|
||||
A relationship is a foreign key linking a **child** table to a **parent**'s
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primary key. In the [example library](/getting-started/example-library/) each
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book is written by one author, so `books` (the child) points at `authors` (the
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parent):
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A relationship is a foreign key from a child table to a parent's primary
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key. In simple mode:
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```rdbms
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add 1:n relationship as books_author from authors.author_id to books.author_id on delete cascade
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```
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You name the relationship with `as <name>` (here `books_author`), then give the
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parent endpoint `from authors.author_id` and the child endpoint
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`to books.author_id`. The `1:n` reads "one author, many books".
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## Viewing a relationship
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`show relationship` draws the two tables and the link between them:
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```rdbms
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show relationship books_author
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```
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```
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┌───────────────────────┐ ┌───────────────────────────┐
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│ books │ │ authors │
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├──────────────┬────────┤ ├──────────────────┬────────┤
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│ book_id (PK) │ serial │ ┌──────1▶│ author_id (PK) ● │ serial │
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│ title │ text │ │ │ name │ text │
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│ author_id ● │ int │n────────┘ │ birth_year │ int │
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│ published │ int │ └──────────────────┴────────┘
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│ isbn │ text │
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└──────────────┴────────┘
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on delete cascade · on update no action
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```
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Read the diagram like this:
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||||
- The **child** (the table holding the foreign key) is on the **left**; the
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**parent** is on the **right**.
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- A filled dot **●** marks each column that takes part in the link.
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- The connector runs **`n` … `1`** — many child rows to one parent row — and
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the **▶** arrowhead points at the parent key.
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- The line beneath records the referential actions (below).
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|
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A table's own `show table` includes a compact **Relationships** section that
|
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summarises every link it takes part in:
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```rdbms
|
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show table books
|
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```
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||||
|
||||
```
|
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books
|
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┌───────────┬────────┬─────────────┐
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│ Name │ Type │ Constraints │
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||||
├───────────┼────────┼─────────────┤
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│ book_id │ serial │ PK │
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│ title │ text │ NOT NULL │
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│ author_id │ int │ │
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│ published │ int │ │
|
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│ isbn │ text │ UNIQUE │
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└───────────┴────────┴─────────────┘
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Relationships
|
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┌─────────────┐ ┌─────────────┐
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│ books │ │ authors │
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├─────────────┤ ├─────────────┤
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│ author_id ● │n───────────────1▶│ author_id ● │
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└─────────────┘ └─────────────┘
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on delete cascade · on update no action
|
||||
```
|
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|
||||
For a one-line summary of every relationship in the project, use `show
|
||||
relationships` (see [Querying & inspecting](/reference/querying-and-inspecting/)).
|
||||
|
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## Referential actions
|
||||
|
||||
`on delete` and `on update` decide what happens to the child rows when a parent
|
||||
row is deleted or its key changes. The action is one of:
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||||
|
||||
| Action | Effect on the children |
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||||
|---|---|
|
||||
| `cascade` | The children are deleted / updated to match. |
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| `set null` | The children's foreign-key column is set to empty. |
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| `restrict` | The change is refused while children still reference the row. |
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|
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With `on delete cascade`, deleting an author removes that author's books too,
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and the result reports each affected relationship:
|
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|
||||
```rdbms
|
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delete from authors where author_id = 3
|
||||
```
|
||||
|
||||
```
|
||||
1 row(s) deleted
|
||||
related: 1 row(s) deleted in `books` for relationship `books_author` (on delete cascade)
|
||||
```
|
||||
|
||||
## Creating the child column at the same time
|
||||
|
||||
If the child column does not exist yet, add `--create-fk` and the playground
|
||||
creates it for you with the right type:
|
||||
|
||||
```rdbms
|
||||
add 1:n relationship as loans_member from members.member_id to loans.member_id --create-fk
|
||||
```
|
||||
|
||||
## Compound keys
|
||||
|
||||
To reference a parent's [compound primary key](/reference/tables/#compound-primary-keys),
|
||||
list the columns on each side, matched in order:
|
||||
|
||||
```rdbms-syntax
|
||||
add 1:n relationship from <Parent>.(<a>, <b>) to <Child>.(<x>, <y>)
|
||||
```
|
||||
|
||||
## Removing a relationship
|
||||
|
||||
```rdbms
|
||||
drop relationship books_author
|
||||
```
|
||||
|
||||
## In advanced mode
|
||||
|
||||
Relationships are ordinary foreign keys, declared with `FOREIGN KEY …
|
||||
REFERENCES` either inline at `CREATE TABLE` or added later with `ALTER TABLE`:
|
||||
|
||||
```sql
|
||||
alter table books add foreign key (author_id) references authors (author_id) on delete cascade
|
||||
```
|
||||
|
||||
## Syntax
|
||||
|
||||
```rdbms-syntax
|
||||
add 1:n relationship [as <Name>]
|
||||
@@ -21,18 +144,5 @@ add 1:n relationship [as <Name>]
|
||||
drop relationship <Name>
|
||||
```
|
||||
|
||||
`<action>` is `cascade`, `set null`, or `restrict`. `--create-fk` adds the
|
||||
child column if it doesn't exist yet.
|
||||
|
||||
**Compound keys.** To reference a parent's compound primary key, list the
|
||||
columns on each side, matched in order:
|
||||
|
||||
```rdbms-syntax
|
||||
add 1:n relationship from <Parent>.(<a>, <b>) to <Child>.(<x>, <y>)
|
||||
```
|
||||
|
||||
In advanced mode, relationships are declared with `FOREIGN KEY … REFERENCES`
|
||||
in `CREATE TABLE` or `ALTER TABLE`.
|
||||
|
||||
See also [Indexes](/reference/indexes/) and [Types](/reference/types/) (why a
|
||||
foreign-key column's type can differ from the key it references).
|
||||
|
||||
Reference in New Issue
Block a user