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Fourth cast: build a minimal two-table schema with rows, switch to advanced mode (`mode advanced`), and run a join pairing each book with its author — shows the mode switch + SQL + multi-table result, motion that complements the guide's static examples. Convert the guide to .mdx and embed above the intro. Recorded via `pnpm casts`; build clean (25 pages).
135 lines
5.8 KiB
Plaintext
135 lines
5.8 KiB
Plaintext
---
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title: Querying with joins
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description: Combine rows from related tables — author with book, member with loan — using SQL joins in advanced mode.
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sidebar:
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order: 2
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---
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import Demo from '../../../components/Demo.astro';
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[Build the library](/guides/build-the-library/) left you with four related
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tables. So far each `show data` looks at one table at a time — but the
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interesting questions span tables: *who wrote this book? who has borrowed it?*
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Answering those means a **join**: matching rows from one table against related
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rows in another.
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<Demo src="/casts/joins.cast" title="Switch to advanced mode, then join books to their authors." />
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Joins are part of SQL, so this guide is in **advanced mode**. Switch to it with
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the `mode` command:
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```rdbms
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mode advanced
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```
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If you only need one SQL command without leaving simple mode, prefix it with a
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colon instead — see [Simple and advanced modes](/getting-started/modes/).
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## Join two tables
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`books` stores an `author_id`, not the author's name. To show the name, join
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`books` to `authors`, matching each book's `author_id` to the author it points
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at:
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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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The `on books.author_id = authors.author_id` clause is the heart of the join:
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it says *which* rows belong together. Because Le Guin wrote two books, her name
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appears on two rows — exactly the one-to-many relationship you declared,
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expanded into result rows.
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## Join through the bridge table
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The real payoff is the many-to-many link. *Who has borrowed which book?* lives
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in three tables: the borrower in `members`, the book in `books`, and the
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connection in `loans`. Join all three, hopping through the `loans` bridge:
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```sql
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select members.name, books.title, loans.loaned_on, loans.returned_on from loans
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join books on loans.book_id = books.book_id
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join members on loans.member_id = members.member_id
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order by loans.loaned_on
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```
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```
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┌──────────────┬──────────────────────┬────────────┬─────────────┐
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│ name │ title │ loaned_on │ returned_on │
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├──────────────┼──────────────────────┼────────────┼─────────────┤
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│ Grace Hopper │ A Wizard of Earthsea │ 2024-05-01 │ (null) │
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│ Alan Turing │ Invisible Cities │ 2024-05-03 │ 2024-05-20 │
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└──────────────┴──────────────────────┴────────────┴─────────────┘
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```
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Each loan row pulls in the matching book *and* the matching member, turning
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three tables of IDs into a readable sentence: who borrowed what, and when.
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## Filter a join
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A `where` clause narrows a join just like it narrows a single table. A loan
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that has not been returned has an empty `returned_on`, so *which books are
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currently out?* is:
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```sql
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select members.name, books.title from loans
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join books on loans.book_id = books.book_id
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join members on loans.member_id = members.member_id
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where loans.returned_on is null
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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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│ Grace Hopper │ A Wizard of Earthsea │
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└──────────────┴──────────────────────┘
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```
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## Summarise with group by
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Joins combine with `group by` to count and total. *How many books has each
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author written?* groups the joined rows by author and counts each group:
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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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`count(*)` counts the rows in each group; `as book_count` names the result
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column. Le Guin's two books collapse into a single row with a count of `2`.
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## Where to go next
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- See **how** a query runs — prefix any of these with `explain` to view the
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plan, and add an [index](/reference/indexes/) to speed up a join.
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- The full `select` surface — projections, more join forms, set operations,
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and CTEs — is in
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[Querying & inspecting](/reference/querying-and-inspecting/).
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- Switch back to simple mode any time by running `mode simple`.
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