# ADR-0035: Advanced-mode SQL DDL

## Status

Accepted. Design agreed with the user (2026-05-24); the approach is
**validated end-to-end by sub-phases 4a / 4a.2 / 4a.3 / 4b / 4c / 4d /
4e / 4f / 4g / 4h / 4i** (`CREATE TABLE` with column- and table-level
constraints and foreign keys, `DROP TABLE [IF EXISTS]`,
`CREATE [UNIQUE] INDEX` / `DROP INDEX [IF EXISTS]`, `ALTER TABLE`
add/drop/rename column, `ALTER TABLE … ALTER COLUMN TYPE`, `ALTER TABLE`
add/drop constraint + add foreign key, `ALTER TABLE … RENAME TO`, and the
4i verification sweep — completion merge, simple/advanced completion
colour, describe of table-level constraints, self-ref FK indicator, and
the CREATE-TABLE help/usage refresh — implemented 2026-05-25/26 — plans
`docs/plans/20260524-adr-0035-sql-ddl-4a.md`, `…-4a2.md`, `…-4a3.md`,
`docs/plans/20260525-adr-0035-sql-ddl-4b.md`, `…-4c.md`, `…-4d.md`,
`…-4e.md`, `…-4f.md`, `…-4g.md`,
`docs/plans/20260526-adr-0035-sql-ddl-4h.md`,
`docs/plans/20260526-adr-0035-sql-ddl-4i.md`). **Phase 4 is complete**
(4a–4i all shipped). **Amendment 1 (2026-05-26)** adds a way to **drop a
composite UNIQUE** via a derived, engine-neutral name (`unique_<cols>`)
that reuses the existing `DROP CONSTRAINT <name>` grammar — no new
syntax, no metadata, the §4g anonymity decision intact (see the amendment
below). This is **Phase 4** of the ADR-0030 roadmap (the
advanced-mode SQL surface), the peer of ADR-0031 (expression grammar),
ADR-0032 (`SELECT`), and ADR-0033 (DML). It **clarifies ADR-0030 §4**
on how DDL is represented and executed.

**Refinements (2026-05-24, pre-implementation `/runda` round,
user-confirmed).** Two open micro-calls were settled before 4a:
(1) `IF [NOT] EXISTS` is **admitted** as a no-op-that-succeeds-with-a-note
rather than refused — it is a near-universal cross-vendor idiom
(PostgreSQL, MySQL/MariaDB, SQLite, Oracle 23ai), not an
engine-specific spelling, so it belongs in the standard surface
(§3/§4/§12/§13); (2) `INTEGER PRIMARY KEY` maps to a **plain `int`**
primary key, *not* auto-increment — `serial` remains the sole
auto-increment type (§3).

## Context

ADR-0030 fixed the *architecture* of advanced mode — SQL authored as
grammar in the unified tree (not a separate batch parser), with the
playground's own type vocabulary and metadata model — and noted that
each large grammar piece gets its own focused ADR. Phases 1–3 shipped:
the SQL expression grammar (ADR-0031), full `SELECT` (ADR-0032), and
DML — `INSERT`/`UPDATE`/`DELETE` (ADR-0033). Phase 4 is **DDL**:
`CREATE` / `DROP` / `ALTER TABLE` and `CREATE` / `DROP INDEX`.

Two things from the earlier phases shape this one:

1. **The advanced surface gets its *own* commands.** ADR-0033
   established that a SQL statement produces a distinct command
   (`SqlInsert` / `SqlUpdate` / `SqlDelete`), separate from the
   simple-mode typed command for the same verb. Those DML commands
   execute as **validated SQL run verbatim** — possible only because
   DML changes no schema and touches no metadata.
2. **DDL cannot run verbatim.** If `CREATE TABLE Orders (id INTEGER)`
   executed as-is, the engine would make the table, but the
   playground would lose what the user meant: that `id` is `serial`,
   that a `REFERENCES` clause is a *named relationship*, that `STRICT`
   applies, that the ten-type vocabulary governs. Recovering that
   needs the parsed statement either way.

ADR-0030 §4 said "DDL → a `Command` … run the typed executor." That
remains right in spirit — DDL is *structurally* executed, not raw —
but it predates the DML build and read as "reuse the simple-mode
`CreateTable` variant." This ADR clarifies it: **DDL gets its own
advanced commands too**, executed structurally (not verbatim). The
"verbatim" execution of the DML commands is an implementation
convenience available only because nothing about DML required
otherwise — not an architectural rule.

Requirements touched: realizes `Q4` for DDL; closes the advanced-mode
side of table/column/index/constraint/relationship operations; lands
the table-rename half of `C1` (advanced mode only).

## Decision

### 1. Own per-statement SQL DDL commands (clarifies ADR-0030 §4)

New `Command` variants, one per statement kind — granularity mirrors
the DML phase:

- `SqlCreateTable`
- `SqlAlterTable`
- `SqlDropTable`
- `SqlCreateIndex`
- `SqlDropIndex`

They are produced by the unified grammar's `ast_builder`s in advanced
mode. Unlike the DML `Sql*` commands they **execute structurally**:
the handler reads the parsed structure and performs the schema change
through the playground's metadata-maintaining machinery — writing
`__rdbms_playground_columns` / `__rdbms_playground_relationships`,
applying `STRICT`, using the ten-type vocabulary — so an
advanced-mode-created object is a first-class playground object,
identical to a simple-mode-created one (ADR-0030 §5).

**Simple mode is untouched.** The existing typed commands
(`CreateTable`, `AddColumn`, `AddRelationship`, …) and their grammar
are unchanged; advanced SQL DDL is purely additive.

**Execution sharing (per the user's steer).** The SQL DDL handlers
**reuse the low-level schema/metadata helpers** — the table builder,
the metadata writers, the rebuild-table primitive (ADR-0013) — where
the underlying operation is genuinely the same, so the two surfaces
cannot drift. Where the SQL path is genuinely different (e.g. a
`CREATE TABLE` that declares several inline foreign keys, which has no
simple-mode shape), it is implemented directly **for clarity rather
than bending the simple-mode command shapes to absorb it**. Shared
where it works; separate where it doesn't.

### 2. Dispatch — shared entry words, advanced-only `alter`

`create` and `drop` are already simple-mode entry words. They reuse
the **category-grouped, mode-aware dispatch** from ADR-0033
Amendment 1: each appears in both the `Simple` and `Advanced` groups
of the `REGISTRY`; in advanced mode the SQL node is tried first and
falls back to the simple node when the SQL shape doesn't match. So in
advanced mode `CREATE TABLE T (id serial)` parses as SQL while
`create table T with pk id(serial)` still parses as the simple form —
exactly as `insert` behaves today. `alter` is a **new advanced-only
entry word** (`CommandCategory::Advanced`); simple mode keeps its
`add column` / `drop column` / `rename column` / `change column`
verbs and gains no `alter`.

### 3. Type vocabulary (restates ADR-0030 §5)

The type-name slot accepts the playground keywords directly (`text`,
`int`, `real`, `decimal`, `bool`, `date`, `datetime`, `blob`,
`serial`, `shortid`) **and** standard-SQL aliases mapped onto them:
`integer`/`smallint`/`bigint` → `int`; `varchar`/`char` → `text`;
`boolean` → `bool`; `timestamp` → `datetime`; `numeric` → `decimal`;
`float`/`double precision` → `real`; `binary`/`varbinary` → `blob`. A
length/precision argument (`varchar(255)`, `numeric(10,2)`) is
**accepted and ignored** — the playground's types are
unparameterised. Engine storage-type names are neither accepted as
input nor shown (§9).

The map is purely **lexical**: `INTEGER PRIMARY KEY` becomes a plain
`int` primary key — it is **not** treated as auto-increment, unlike
the engine's rowid-alias idiom. Auto-increment is reached only through
the explicit `serial` type (`id serial primary key`). This keeps the
engine's storage behaviour from leaking into the standard surface and
matches ADR-0005's single-auto-increment-type model.

### 4. The DDL surface (full; `Q4`, no pre-emptive cuts)

**`CREATE TABLE <name> ( <element>, … )`**

- **Column elements**: `<name> <type> [constraints…]`, where the
  column constraints are the ADR-0029 set spelled in SQL: `NOT NULL`,
  `UNIQUE`, `PRIMARY KEY`, `DEFAULT <expr>`, `CHECK (<expr>)`, and an
  inline `REFERENCES <T>(<col>) [ON DELETE …] [ON UPDATE …]` (§5).
- **Table elements**: `PRIMARY KEY (<col>, …)` (single **and
  compound**), `UNIQUE (<col>, …)`, `CHECK (<expr>)`,
  `[CONSTRAINT <name>] FOREIGN KEY (<col>) REFERENCES <T>(<col>)
  [ON DELETE …] [ON UPDATE …]` (§5).
- `CHECK` and `DEFAULT` expressions reuse the ADR-0031 `sql_expr`
  grammar (the same fragment `WHERE`/`HAVING`/projections use).
- `CREATE TABLE IF NOT EXISTS <name> …` is admitted: when the table
  already exists the statement is a **no-op that succeeds with a note**
  ("table already exists — skipped") instead of the plain-form
  "table already exists" error. `IF NOT EXISTS` is a near-universal
  cross-vendor idiom, not an engine-specific spelling, so it is part of
  the standard surface (refines §12).

**`DROP TABLE [IF EXISTS] <name>`** → `SqlDropTable`. Cascade of inbound
relationships follows the existing `drop table` semantics. `IF EXISTS`
is admitted (universal across the major engines): dropping an absent
table is then a **no-op that succeeds with a note** instead of the
plain-form "no such table" error.

**`ALTER TABLE <name> <action>`** → `SqlAlterTable`, where `<action>`
covers, mapping to the existing low-level operations:

| SQL action | Underlying operation |
|---|---|
| `ADD COLUMN <name> <type> [constraints]` | add-column (ADR-0013 rebuild where needed) |
| `DROP COLUMN <name>` | drop-column |
| `RENAME COLUMN <old> TO <new>` | rename-column |
| `ALTER COLUMN <name> TYPE <type>` | change-column-type (§5 conversion) |
| `ADD [CONSTRAINT <name>] <table-constraint>` | add-constraint / add-relationship (FK) |
| `DROP CONSTRAINT <name>` | drop-constraint |
| `RENAME TO <new>` | **table rename (§6, new low-level op)** |

**`CREATE [UNIQUE] INDEX [<name>] ON <table> (<col>, …)`** →
`SqlCreateIndex`, mapped to the ADR-0025 index machinery; `UNIQUE`
sets the index's uniqueness (a small extension to ADR-0025's index
model if it does not already carry the flag, called out in §13).

**`DROP INDEX <name>`** → `SqlDropIndex`.

### 5. Foreign keys → named relationships

A `REFERENCES` / `FOREIGN KEY` clause is the SQL spelling of an
ADR-0013 relationship. Because `SqlCreateTable` is its own command
carrying the whole parsed structure, a `CREATE TABLE` that declares
FK columns **creates the table and its relationship metadata
together** — one statement, one command, one transaction, **one undo
step** (§10). No decomposition into separate commands is needed.

- `ON DELETE` / `ON UPDATE` → the ADR-0013 referential actions.
- A `CONSTRAINT <name> FOREIGN KEY …` names the relationship; an
  unnamed FK is auto-named by the existing ADR-0013 convention.
- `ALTER TABLE child ADD [CONSTRAINT <name>] FOREIGN KEY (<col>)
  REFERENCES <P>(<col>) …` adds a relationship to an existing table
  (the clean 1:1 with add-relationship).
- FK column type compatibility follows `Type::fk_target_type`
  (ADR-0011) unchanged.

### 6. Table rename — advanced mode only (`C1`)

`ALTER TABLE <old> RENAME TO <new>` is **advanced-mode only**; there
is no simple-mode rename-table verb. It needs a genuinely new
low-level operation (none exists today): within one transaction,
rename the table in the database, rename its `data/<table>.csv` file,
and update every metadata row that names it — the column-metadata
rows, **both ends of any relationship** in
`__rdbms_playground_relationships` that references the old name, and
**the table-level CHECK rows** in `__rdbms_playground_table_checks`
(added in 4a.3; keyed by `table_name`). Name validation and
`__rdbms_*` rejection apply to the target. This closes the rename half
of `C1` for the advanced surface.

### 7. Column type conversion — one engine, mode-appropriate policy

The per-cell classification of ADR-0017 (clean / lossy / incompatible,
plus static refusals for playground-type-specific targets such as
`→ serial` and `↔ blob`) is a property of the **type set**, shared by
both modes. The policy on the *lossy* tier differs by mode:

| Tier | Simple mode | Advanced mode (`ALTER COLUMN … TYPE`) |
|---|---|---|
| **clean** | auto-convert | auto-convert |
| **incompatible** | refuse (friendly) | refuse (friendly) — real SQL errors too |
| **static-refused** (`→serial`, `↔blob`, …) | refuse | refuse — our own types have no SQL meaning to mirror |
| **lossy** (`3.14`→`3`) | **refuse by default**; `--force-conversion` opts in | **perform it** (what SQL does), with a post-op "N values converted with loss" note; **no force flag** |

Rationale: **simple mode protects up front; advanced mode trusts the
user like SQL does and lets `undo` catch regrets.** A lossy advanced
conversion is snapshotted (§10), so it is one `undo` away — there is
no silent *irreversible* loss, and no need to drop to simple mode to
"force". Conversions that exist only in the playground's vocabulary
stay protected in both modes. The simple-mode `--force-conversion` /
`--dont-convert` flags are unchanged and have **no SQL spelling**
(advanced mode always performs the conversion); the Postgres `USING
<expr>` clause is **not** adopted (§12).

### 8. Constraints

Column- and table-level constraints map to the ADR-0029 model:
`NOT NULL`, `UNIQUE`, `PRIMARY KEY` (incl. compound, table-level),
`DEFAULT <expr>`, `CHECK (<expr>)`. A populated-column constraint
addition reuses ADR-0029's pre-flight dry-run guard. `CHECK` /
`DEFAULT` expressions are stored as the SQL the user could re-enter in
advanced mode (ADR-0030 §11) — one syntax, not a third.

### 9. Engine neutrality (ADR-0030 §7)

No engine type names in or out (§3). `STRICT` is applied internally by
the create path; it is not in the authored grammar, so typing it is an
ordinary parse error, not a surfaced engine feature. Parse errors,
out-of-subset refusals, and execution failures route through the
friendly-error layer (ADR-0019) with engine-neutral wording.

### 10. Persistence, metadata, history, replay, undo

- Structural execution keeps `project.yaml`, the metadata tables, and
  the CSV layer correct with the same guarantees as the simple-mode
  path (ADR-0015 §6 ordering preserved).
- `history.log` records the **literal submitted SQL line**; replay
  re-runs it through the one walker with the advanced view active.
  `create` / `drop` / `alter` are **schema-write entry words, not in
  ADR-0034 Amendment 1's app-lifecycle skip set**, so SQL DDL
  **replays as a write** (re-applied) with **no replay-filter change**
  — unlike `undo` / `redo`, which had to be added to that skip set.
- **Undo (ADR-0006):** each SQL DDL statement is a user mutation
  carrying a `source`, so it is snapshotted by the worker hook and is
  **one undo step** — including a `CREATE TABLE` with foreign keys,
  precisely because it is a single command (§5) rather than a
  decomposed sequence.

### 11. Ambient assistance comes for free (ADR-0030 §8)

Because the DDL is grammar in the unified tree, the walker
**mechanisms** apply with no DDL-specific assistance code: syntax
highlighting, the `[ERR]`/`[WRN]` validity indicator (ADR-0027), the
per-command parse-error usage skeleton (ADR-0021), and the completion
engine.

What each grammar node still **authors** (this is writing the grammar,
not bolting assistance on afterwards): the correct `IdentSource` on
every schema-name slot — so `ALTER TABLE`/`DROP TABLE`/`DROP INDEX`
and `REFERENCES T(col)` / `CREATE INDEX ON T (cols)` complete from the
`SchemaCache`; the per-node hint + usage catalog keys (as the
app-command nodes carry `help_id` / `usage_ids`); and the
DDL-specific walker diagnostics with their catalog keys — the DDL
peers of the DML diagnostics ADR-0033 added (e.g. unknown type,
column-already-exists, FK column-type mismatch, the §7 lossy-conversion
note). The integration is structural, not free of authoring.

### 12. Out of scope

- Per ADR-0030 §3: views, triggers, transaction control, `PRAGMA`,
  `ATTACH`/`DETACH`, `VACUUM`, virtual tables, multi-statement
  batches. One statement per submission; a trailing `;` is tolerated.
- The Postgres `USING <expr>` conversion clause (§7) — heavy
  (per-row expression evaluation), dialect-specific, and unable to
  express playground-type targets.
- The simple-mode `--dont-convert` semantics have no SQL form
  (advanced `ALTER COLUMN TYPE` always converts).
- The **DSL → SQL teaching echo** (ADR-0030 §10) is Phase 5, a
  separate ADR — not this one.
- Engine-specific DDL spellings (`AUTOINCREMENT`, `WITHOUT ROWID`,
  collations) — the grammar admits the standard surface; extras are
  ordinary parse errors. (`IF [NOT] EXISTS` was **reclassified into
  scope** — see §4 — as a near-universal cross-vendor idiom rather
  than an engine-specific spelling.)

### 13. Phased implementation plan

Sub-phases, each opening with the smallest end-to-end slice and each
with an explicit exit gate + a written Devil's-Advocate gate, mirroring
ADR-0033's structure:

- **4a — Dispatch + `CREATE TABLE` core.** Advanced `create`
  dispatch; `SqlCreateTable` for columns + types (the §3 map, incl. the
  two-word `double precision` and discarded length args) + the
  **clean-reuse column constraints only** — `NOT NULL` / `UNIQUE` /
  column-level `PRIMARY KEY` — + single/compound table-level
  `PRIMARY KEY`, plus `IF NOT EXISTS` (no-op-with-note, §4). Reuses
  `do_create_table`, whose inline-PK rule is aligned with the rebuild
  generator `schema_to_ddl` (inline only a first-column single PK) so a
  created table and its rebuilt form have identical DDL; `serial`
  autoincrement is independent of inline-vs-table-level PK (the insert
  path computes the next value), verified by round-trip tests. **No
  FK** (4b); **no `DEFAULT`/`CHECK`/table-level `UNIQUE`** (4a.2).
- **4a.2 — Per-column `CHECK`/`DEFAULT` + composite `UNIQUE(a,b)`.**
  Split out (2026-05-24) and re-scoped (2026-05-25, user-confirmed) to
  the constraints that need **no new internal table**: (1)
  **`CHECK`/`DEFAULT`** via the full `sql_expr` surface stored as **raw
  SQL text** — `sql_expr` is validate-only (no `Expr` AST for
  `compile_check_sql`/`ColumnSpec`), so a separate execution path
  captures the raw expression text; per-column `CHECK` reuses the
  existing `__rdbms_playground_columns.check_expr` column, `DEFAULT`
  round-trips via the engine's native `PRAGMA table_info`; (2)
  **composite `UNIQUE(a,b)`** — a new `TableSchema.unique_constraints`
  field, detected on read via the UNIQUE-constraint index
  (`PRAGMA index_list` origin `u`), round-tripped through YAML, with
  save/load/rebuild tests.
- **4a.3 — Table-level / multi-column `CHECK(…)`.** *(Implemented
  2026-05-25 — plan `docs/plans/20260525-adr-0035-sql-ddl-4a3.md`.)*
  Split from 4a.2 (2026-05-25, user-confirmed) because SQLite exposes
  **no PRAGMA for CHECK constraints**, so a table-level CHECK cannot be
  read back from the engine and needs a **new `__rdbms_*` metadata
  table** as its source of truth (the ADR-0012/0013 pattern) — a
  distinct architectural step. Landed as
  `__rdbms_playground_table_checks (table_name, seq, check_expr)`; the
  builder distinguishes a table-level CHECK from a column-level one by
  element position (no column-def open). Composite `UNIQUE` deliberately
  stays PRAGMA-detected (engine-reportable, unlike CHECK). (The general
  rule: a DDL feature needs new model/execution work only when it
  introduces a structure simple mode could never produce, or an
  expression the structural helper cannot consume — cf. the
  `UNIQUE`-index flag in 4d and the rename op in 4h.)
- **4b — Foreign keys in `CREATE TABLE`.** *(Implemented 2026-05-25 —
  plan `docs/plans/20260525-adr-0035-sql-ddl-4b.md`.)* Inline
  `REFERENCES <parent>[(<col>)] [ON DELETE/UPDATE …]` + table-level
  `[CONSTRAINT <name>] FOREIGN KEY (<col>) REFERENCES …` → ADR-0013
  relationship metadata, written in the create transaction (one undo
  step). Reuses the relationship name/uniqueness/metadata helpers
  shared with `add relationship`; `do_create_table` emits the
  `FOREIGN KEY` clause identically to `schema_to_ddl`. **Self-references**
  (parent = the table being created, validated against the in-statement
  columns/PK) and the **bare `REFERENCES <parent>`** form (resolves to
  the parent's single-column PK; composite → error) are both supported
  (user-confirmed). Inline FKs are auto-named; only the table-level form
  takes `CONSTRAINT <name>`. PK-target only (UNIQUE-target deferred with
  `add relationship`); `Type::fk_target_type` (ADR-0011) governs type
  compatibility.
- **4c — `DROP TABLE [IF EXISTS]`** → `SqlDropTable`. *(Implemented
  2026-05-25 — plan `docs/plans/20260525-adr-0035-sql-ddl-4c.md`.)*
  Reuses `do_drop_table` (cascade parity + the inbound-relationship
  refusal + metadata cleanup), so it matches the simple `drop table`;
  `IF EXISTS` on an absent table is a no-op-with-note (a new
  `DropOutcome::Skipped` mirroring `CreateOutcome::Skipped`; journalled,
  no snapshot, §4). `drop` is a shared entry word: `drop table` parses
  as `SqlDropTable` in advanced mode, `drop column`/`relationship`/
  `index`/`constraint` fall back to the simple `drop` node. Advanced-
  mode `drop ` completion now surfaces the SQL `table` (the
  shared-entry-word behaviour from `create`, ADR-0033 Amendment 3); the
  DSL drops still parse via fallback — 4i grows the surface as `DROP
  INDEX` lands in 4d.
- **4d — `CREATE [UNIQUE] INDEX` / `DROP INDEX`** → `SqlCreateIndex`
  / `SqlDropIndex`. *(Implemented 2026-05-25 — plan
  `docs/plans/20260525-adr-0035-sql-ddl-4d.md`.)* Both reuse the ADR-0025
  executors (`do_add_index` / `do_drop_index`), like 4c reused
  `do_drop_table`. `CREATE [UNIQUE] INDEX [IF NOT EXISTS] [<name>] ON <T>
  (cols)` (the unnamed form auto-named per ADR-0025; the leading
  `[UNIQUE]` is a concrete-keyword `Choice`, the optional name an
  `on`-led-first selector mirroring the `drop index` positional
  selector) and `DROP INDEX [IF EXISTS] <name>` (name-only — the
  positional `drop index on T(…)` stays the simple form via fallback).
  `IF [NOT] EXISTS` reuses the 4c no-op-with-note skip (journalled, not
  snapshotted). **`CREATE UNIQUE INDEX` is admitted** (user-confirmed
  2026-05-25): ADR-0025 deferred UNIQUE indexes for the *simple-mode
  DSL*, but advanced mode "trusts the user like SQL does" (§7) — so the
  model gains an `IndexSchema.unique` flag (additive YAML, `version` 1;
  rebuild re-emits `CREATE UNIQUE INDEX`; the structure view + items
  panel mark `[unique]`), recorded as **ADR-0025 Amendment 1**.
  Simple-mode `add unique index` stays deferred. `create`/`drop` each
  gain a *second* advanced node, exercising the all-candidates dispatch
  (`decide` tries every advanced candidate).
- **4e — `ALTER TABLE` add/drop/rename column.** *(Implemented
  2026-05-25 — plan `docs/plans/20260525-adr-0035-sql-ddl-4e.md`.)*
  `alter` is a new advanced-**only** entry word (like `select`/`with`);
  `ALTER TABLE <T> ADD COLUMN <col> <type> [NOT NULL|UNIQUE|DEFAULT|
  CHECK] | DROP COLUMN <col> | RENAME COLUMN <old> TO <new>` →
  `SqlAlterTable { AlterTableAction }`, **runtime-decomposed** to the
  existing `do_add_column` / `do_drop_column` / `do_rename_column` (one
  undo step each) — no new worker layer. The `COLUMN` keyword is
  required (reserves bare `RENAME TO` for 4h, `ADD CONSTRAINT` for 4g);
  ADD COLUMN takes column constraints only (no PK / inline REFERENCES).
  **`do_add_column` was extended** to consume the SQL raw-text
  `default_sql` / `check_sql` (DEFAULT/CHECK; `sql_expr` is validate-only
  — the 4a.2 mechanism), so ADD COLUMN reaches parity with `CREATE
  TABLE`'s column constraints. Drop/rename column now **refuse a column a
  CHECK references** — the 4a.3 deferral, extended (user-confirmed) to
  **both table-level and column-level CHECKs** — detected up-front by
  tokenizing the raw CHECK text (`check_references_column`, skipping
  string literals); for RENAME the column's *own* column-level CHECK
  counts (it drifts too), for DROP it does not (it drops with the
  column). This lives in the shared executors, so it guards **both** the
  simple `drop/rename column` and the SQL surface, fixing a latent
  rename-drift bug (a native rename rewrites the live CHECK but leaves the
  stored text — table-level in `__rdbms_playground_table_checks` or
  column-level in `__rdbms_playground_columns` — stale, breaking a later
  rebuild). SQL
  `DROP COLUMN` over an index-covered column is **refused** (no
  `--cascade` SQL spelling — matches SQLite + the simple default;
  user-confirmed). The shared column executors (and `do_add_index`) also
  gained an internal-`__rdbms_*`-table guard (refuse as "no such table"),
  closing a pre-existing exposure on both surfaces (user-confirmed). The
  friendly wording of the CHECK-guard refusal is **H1**. *(The broader
  internal-table guard on `do_change_column_type` / `do_add_constraint` /
  `do_add_relationship` is a tracked follow-up.)*
- **4f — `ALTER TABLE … ALTER COLUMN TYPE`** (the §7 conversion
  model + the lossy-with-note path). *(Implemented 2026-05-25 — plan
  `docs/plans/20260525-adr-0035-sql-ddl-4f.md`.)* A fourth
  `AlterTableAction::AlterColumnType`, runtime-decomposed to the existing
  `change_column_type` executor with `ChangeColumnMode::ForceConversion`
  — which **is** the §7 advanced policy: lossy cells are *performed* and
  counted (the engine-neutral `client_side.transformed_lossy` note
  fires), incompatible cells refuse, and the ADR-0017 static refusals
  (`↔ blob`, same-type, `date ↔ datetime`, non-`int → serial`) refuse in
  both modes. **`int → serial` is *allowed*** (auto-fills nulls, adds
  UNIQUE if non-PK — ADR-0018 §8; the §7 "static-refused →serial"
  summary is looser than the code). No force flag, no `USING`, no
  `SET DATA TYPE` synonym (§7/§12); `undo` is the advanced safety net.
  The grammar adds a fourth action branch leading on `alter`,
  discriminated in the builder by the **`type` keyword** (unique — ADD
  COLUMN's type is an ident); the type slot reuses `SQL_TYPE`. The
  internal-`__rdbms_*` guard was folded into `do_change_column_type`
  (user-confirmed 2026-05-25), closing the simple `change column`
  exposure too. *(The remaining internal-table guard on
  `do_add_constraint` / `do_add_relationship` rides in 4g.)*
- **4g — `ALTER TABLE` add/drop constraint, add foreign key.**
  *(Implemented 2026-05-25 — plan
  `docs/plans/20260525-adr-0035-sql-ddl-4g.md`.)* `ALTER TABLE <T> ADD
  [CONSTRAINT <name>] (CHECK (…) | UNIQUE (…) | FOREIGN KEY (…)
  REFERENCES …)` and `DROP CONSTRAINT <name>`. **ADD scope (user-
  confirmed):** CHECK + composite UNIQUE + FK; `ADD PRIMARY KEY` is
  refused (every table already has a PK) and a **named UNIQUE** is
  refused (composite UNIQUE is anonymous in our model — PRAGMA-detected,
  §4a.2). Each ADD reuses an existing low-level path: table-CHECK and
  composite-UNIQUE rebuild the table (dry-run guards reject existing
  rows that would violate), FK decomposes to `add_relationship` (the
  same machinery `add 1:n relationship` uses — bare `REFERENCES <P>`
  resolves to the parent's single PK; `create_fk = false` as the column
  must exist). **DROP CONSTRAINT (user-confirmed)** resolves the name to
  a named table-CHECK then a named FK whose child is `<T>`, else refuses.
  **Named table-CHECK round-trip (user-confirmed):** the `CHECK_TABLE`
  metadata gains a nullable `name` column (**rebuild-only** arrival — a
  pre-4g project gains it on `rebuild`; a named CHECK add on an
  un-upgraded project is refused with a friendly "rebuild first"
  message), and `project.yaml`'s `check_constraints` is **extended** to
  carry the name (`{expr, name}` mapping; the bare-string form still
  reads, name = `None`) so a named CHECK survives a rebuild — `rebuild`
  reconstructs from the yaml. The internal-`__rdbms_*` guard was folded
  into `do_add_constraint` / `do_add_relationship`, completing the
  4d/4e/4f guard class. One undo step per statement.
- **4h — `ALTER TABLE … RENAME TO`** (the §6 new low-level op).
  *(Implemented 2026-05-26 — plan
  `docs/plans/20260526-adr-0035-sql-ddl-4h.md`.)* The one genuinely new
  low-level executor in Phase 4 (`do_rename_table`): a native engine
  `RENAME TO` (structure-preserving — no rebuild) plus reconciliation, in
  one transaction (commit-db-last), of everything the engine does not
  track — every metadata row that names the table
  (`__rdbms_playground_columns`, **both ends** of
  `__rdbms_playground_relationships`, `__rdbms_playground_table_checks`),
  the CSV file (via the existing persistence rewrite+delete path:
  `rewritten_tables = [new]`, `deleted_tables = [old]` — no new
  persistence method), and **CHECK text that qualifies a column with the
  old table name** (`T.age` → `U.age`, both column- and table-level — a
  planning-`/runda` finding: the engine rewrites the *live* CHECK but the
  *stored* text would drift and break a fresh rebuild;
  `rewrite_check_table_qualifier` keeps them in step; bounded because a
  CHECK references only its own table). Grammar: a fifth action,
  `AlterTableAction::RenameTable { new }`, added by splitting the `rename`
  verb into one branch with an inner `Choice` on a distinct second keyword
  (`column` → rename-column, `to` → rename-table — the §6.1 trap-safe
  pattern); the new-name slot mirrors the `CREATE TABLE` name slot
  (`IdentSource::NewName` + the `reject_internal_table` parse validator).
  **Refusals (user-confirmed 2026-05-26):** rename to the same name, to an
  existing other table, to an `__rdbms_*` name, or of a non-existent
  table. Collision checks are **case-insensitive** (the engine matches
  names that way), with an engine-neutral pre-check so a case-only rename
  or a case-insensitive clash never surfaces the raw engine error
  (a finished-slice `/runda` finding). **Auto-named indexes *and*
  relationships keep their stale names** (only the table-name *columns*
  update; ADR-0035 §6 scope — user-confirmed; documented collision
  caveat). One undo step (the whole-project snapshot). Advanced-mode only;
  closes the rename half of `C1`.
- **4i — Verification sweep (completes Phase 4).** *(Implemented
  2026-05-26 — plan `docs/plans/20260526-adr-0035-sql-ddl-4i.md`.)*
  - **(a) `CREATE TABLE` help/usage skeleton** refreshed for the 4a.2
    `DEFAULT`/`CHECK`/composite-`UNIQUE`, 4a.3 table-`CHECK`, and 4b FK
    forms (the index forms already carried their own since 4d).
  - **(b) `describe` of table-level constraints** — `TableDescription`
    gained `unique_constraints` + `check_constraints`, rendered in a
    "Table constraints:" section (composite `UNIQUE`, table `CHECK`
    incl. **named** CHECKs). The per-column `[unique]`-index marker
    shipped in 4d.
  - **(c) self-ref FK indicator** — `schema_existence_diagnostics`
    collects the `CREATE TABLE` target(s) (`IdentSource::NewName`, role
    `table_name`) and exempts a `Tables` reference matching one from the
    unknown-table flag, so a self-referencing FK no longer pre-flags the
    not-yet-created table; a FK to a genuinely-unknown *other* table
    still flags.
  - **(d) shared-entry-word completion merge** — at the advanced-mode
    entry-word boundary, `completion_probe_in_mode` walks every candidate
    node and unions the viable (`Incomplete`) ones' continuations, so
    `drop ` offers `table·index·column·relationship·constraint` and `drop
    rel` → `relationship` (was an empty dead-end). Completion-only (the
    parse path is untouched); deeper positions keep the committed walk.
  - **(e) simple-vs-advanced completion colour** — each continuation is
    classified `Both`/`Advanced`/`Simple` and, **only when the candidate
    list mixes modes**, coloured (`mode_advanced`/`mode_simple`, `Both` =
    token-kind) and block-ordered `Both → Advanced → Simple` (user-chosen
    design, 2026-05-26). Single-mode lists keep the token-kind colours.
  - **Staples:** matrix/typing-surface coverage extended (completion +
    describe tests); engine-neutral wording held (the vocab audit covers
    the new strings); undo-parity is **N/A for 4i** — every change is
    read-side (completion / diagnostics / describe / help), so no undo
    steps are introduced.

## Amendment 1 — Dropping a composite UNIQUE constraint (2026-05-26)

A whole-Phase-4 `/runda` surfaced that a composite `UNIQUE(a,b)` — kept
**anonymous** by design (§4a.2, and §4g refused a *named* UNIQUE add) —
has **no way to be dropped**: `DROP CONSTRAINT <name>` (§4g) resolves
only a named table-CHECK or a named FK, so recreating the table was the
only escape. This amendment adds a drop path. Written with explicit user
approval; the plan is
`docs/plans/20260526-adr-0035-composite-unique-drop-f1f2f3.md`.
**Implemented 2026-05-26.**

**It does not reverse the §4g anonymity decision.** Storage stays a bare
column-list (`unique_constraints: Vec<Vec<String>>`, PRAGMA-detected) and
a UNIQUE still **cannot be named on `ADD`**. The addition is purely a
**derived, engine-neutral name** used to *display* and *address* the
constraint on drop.

### The derived name (user-decided: derived, no storage; `unique_<cols>`)

The name is a deterministic function of the column list —
`unique_<col1>_<col2>…` — recomputed live wherever it is shown or
matched. Nothing is persisted: the constraint remains a bare column-list,
so the name round-trips for free and needs no metadata table and no
rebuild-arrival migration (the cost §4a.3 deliberately avoided). If a
column in the UNIQUE is later renamed, the displayed name tracks it —
arguably more correct than a frozen stored name. Alternatives weighed and
rejected: naming UNIQUEs with a user-supplied name + new metadata table
(reverses §4g; heaviest), and a positional `drop … unique (cols)` form
(needs new grammar). The derived name **reuses the existing `DROP
CONSTRAINT <name>` grammar — no new syntax**.

### Surfaces

- **`describe` / structure view.** The "Table constraints:" section (4i b)
  annotates each composite UNIQUE with its name: `unique_b_c: UNIQUE
  (b, c)`.
- **`ALTER TABLE <T> DROP CONSTRAINT <name>`** (advanced-SQL only, matching
  the §4g `ADD` form). `do_drop_constraint_by_name` gains a **third
  resolution step** after named table-CHECK and named FK: it recomputes
  the derived name of each composite UNIQUE on `<T>` and matches. On a
  single match it rebuilds the table without that entry (the
  `do_alter_add_unique` rebuild in reverse). **A name matching more than
  one UNIQUE is refused as ambiguous** (e.g. a column literally named
  `b_c` colliding with `UNIQUE (b, c)`) — it never guesses which to drop.
  **Resolution order** means a user-named CHECK/FK with the same string
  shadows a derived UNIQUE name; the distinctive `unique_` prefix makes
  this unlikely and it is documented, not guarded.

### Dropping a *column* a composite UNIQUE covers (F1)

`do_drop_column` gains an up-front guard (alongside the index-covering
and CHECK guards): a column participating in any composite UNIQUE is
refused with the constraint's derived name and the actionable drop
command — `cannot drop \`T.c\` … part of the UNIQUE constraint
\`unique_b_c\` (b, c); drop that constraint first (\`alter table T drop
constraint unique_b_c\`)`. The refusal itself is unchanged (the engine
already refuses it); the message becomes engine-neutral and actionable.
Single-column UNIQUE column drops are a **parallel** gap (different
mechanism — ADR-0029 column-level `drop constraint`) and are **out of
scope** here.

## Amendment 2 — Standard-first dialect + `ALTER COLUMN` constraint gap-fill (2026-05-27)

Designing the **DSL → SQL teaching echo** (ADR-0030 §10, specified in
ADR-0038) surfaced two related things about this ADR's surface. First, a
**dialect drift**: ADR-0030 frames advanced mode as "the **standard-SQL**
surface," but §4f shipped the type-change verb as bare
`ALTER COLUMN … TYPE` — the **PostgreSQL shorthand** — and explicitly
declined the ISO `SET DATA TYPE` synonym (§4f, line "no `SET DATA TYPE`
synonym"). Second, **gaps**: advanced mode has no way to toggle `NOT NULL`
or `DEFAULT` on an existing column, though simple mode does
(ADR-0029 `add`/`drop constraint`), and the rebuild primitive that would
back them is already in place (it backs §4f type-change and §4g
constraint-add). This amendment records a **dialect stance** and **fills
the clean gaps** so the echo can emit portable SQL that is also runnable
in advanced mode. Recorded with explicit user approval (2026-05-27).

### The dialect stance — standard-first (refines ADR-0030)

Where ISO SQL provides a spelling, the **authored grammar's canonical
form is the ISO one**, and the **echo emits the ISO form**. A widely-
recognised vendor shorthand *may* be **accepted** as a synonym (so a
learner who knows it is not punished), but it is never the canonical or
emitted form. Where ISO provides **no** spelling for an operation the
playground teaches, **one** widely-recognised vendor spelling is adopted
as a **deliberate, documented extension** — not silently. This realigns
the surface with ADR-0030's stated posture and makes the divergence a
conscious, recorded choice rather than drift.

The stance applies to the whole advanced surface going forward; this
amendment exercises it on the `ALTER COLUMN` family.

### Type change: ISO `SET DATA TYPE` canonical, `TYPE` retained as a synonym

Reverses §4f's "no `SET DATA TYPE` synonym." The grammar now accepts
**both** `ALTER COLUMN <c> SET DATA TYPE <type>` (ISO; canonical) and
`ALTER COLUMN <c> TYPE <type>` (PostgreSQL; accepted synonym, no
breakage for already-shipped usage). Both decompose to the same §4f
`AlterColumnType` action and the same `ChangeColumnMode::ForceConversion`
executor — semantics are **unchanged**; only the accepted spelling set
and the *canonical/echoed* form change. The echo (ADR-0038) emits
`SET DATA TYPE`.

### New: `SET/DROP DEFAULT` (ISO) and `SET/DROP NOT NULL` (the one extension)

Four new `AlterColumnType`-family actions under `ALTER COLUMN <c>`:

| Spelling | Standing | Decomposes to |
|---|---|---|
| `SET DEFAULT <expr>` | **ISO-standard** | `do_set_column_default` (raw-SQL — see note) |
| `DROP DEFAULT` | **ISO-standard** | `do_drop_constraint(Default)` |
| `SET NOT NULL` | **documented extension** | `do_add_constraint(NotNull)` |
| `DROP NOT NULL` | **documented extension** | `do_drop_constraint(NotNull)` |

**Implementation correction (2026-05-27, during build).** The draft said
`SET DEFAULT` decomposes to `do_add_constraint(Default)`. It cannot:
`do_add_constraint`'s `Constraint::Default(Value)` carries a *typed*
`Value` (the simple-mode shape), but advanced `SET DEFAULT <expr>` is
**raw `sql_expr` text** with no AST (it may be `(1 + 1)`, a function
call, …). So `SET DEFAULT` decomposes to a small dedicated executor
**`do_set_column_default(table, column, default_sql)`** that sets the
column's raw `default_sql` and rebuilds — mirroring `do_add_constraint`'s
`Default` branch (the §6 `serial`/`shortid` refusal, no dry-run — a
default never touches existing rows) but accepting raw SQL instead of a
`Value`. `SET NOT NULL` / `DROP NOT NULL` / `DROP DEFAULT` reuse the
ADR-0029 `do_add_constraint` / `do_drop_constraint` executors unchanged.

`SET DEFAULT`/`DROP DEFAULT` are taken directly from the ISO
`<alter column action>` set. **`NOT NULL` toggling has no ISO spelling**
— in the standard `NOT NULL` is a column constraint, not an in-place
`ALTER COLUMN` verb, and the vendors diverge (PostgreSQL
`SET/DROP NOT NULL`; SQL Server `ALTER COLUMN <c> <type> NOT NULL`;
MySQL `MODIFY`; Oracle `MODIFY`). Per the stance, **one** spelling is
adopted as a deliberate extension: **PostgreSQL's `SET/DROP NOT NULL`**,
chosen because it is the only form that is type-independent (it does not
force the user to restate the column type), reads as plain English, and
composes uniformly with the ISO `SET/DROP DEFAULT` it sits beside.

`SET DEFAULT`'s value slot reuses the §4a.2 / §4e **raw `sql_expr`**
default mechanism (`default_sql`), so a default may be any expression
the create-table `DEFAULT` accepts — one syntax, not a third
(ADR-0030 §11).

### Execution — rebuild-backed, no new low-level op

Each new action **runtime-decomposes to an existing ADR-0029 executor**
(`do_add_constraint` / `do_drop_constraint`), exactly as §4e/§4f
decompose their actions — the populated-column **pre-flight dry-run
guard** (ADR-0029 §5) and the internal-`__rdbms_*` guard come for free.
No new worker layer. The grammar discriminates the `ALTER COLUMN <c> …`
tail by its leading keyword: `type` / `set data type` (type change),
`set not null` / `drop not null`, `set default` / `drop default` — the
`set`/`drop` lead is new alongside §4f's `type` lead.

### Parity reached, and the one residual gap

This brings advanced mode to **constraint-modification parity with
simple mode (ADR-0029) for `NOT NULL` and `DEFAULT`** — add and drop,
both directions. It closes the simple↔advanced asymmetry the echo design
flagged for those ops.

**Residual gap (deliberately not closed here):** dropping a
**column-level `UNIQUE` or `CHECK`** (the single-column, *anonymous*
constraints simple mode adds via ADR-0029 `add constraint unique/check`).
`DROP CONSTRAINT <name>` (§4g) + the derived composite-UNIQUE name
(Amendment 1) resolve *table-level* / *named* constraints; a single-
column column-level `UNIQUE` lives as the column's `unique` flag and a
column-level `CHECK` is likewise anonymous, so neither has a portable
name to address. This is the same class Amendment 1 called a "parallel
gap … out of scope." Consequently ADR-0038's catalogue marks
`drop constraint unique/check from T.col` as **no headline echo** (a
residual gap), rather than inventing a name or a recipe. Flagged for the
user; closing it (e.g. extending the derived-name approach to single-
column UNIQUE) would be its own small follow-up.

### Engine neutrality holds (the rebuild stays hidden)

The chosen spellings are **portable SQL**, not engine features. The fact
that *this* engine satisfies `SET NOT NULL` / `SET DATA TYPE` via a table
rebuild (because it lacks in-place `ALTER`) is a **Category-1 engine
implementation detail** (ADR-0038's taxonomy) and stays **invisible** —
no recipe, no rebuild steps surfaced — exactly as §9 and ADR-0030 §7
require. A learner sees the standard statement; the engine's means of
honouring it is not the lesson.

## Consequences

- Advanced mode reaches DDL parity with simple mode and adds
  table-rename, so a learner can build and evolve a whole schema in
  standard SQL with the playground's types, metadata, and safety
  intact.
- The command set grows by five `Sql*` DDL variants; the worker gains
  their handlers, which lean on shared low-level helpers where the
  operation matches the simple-mode path and stand alone where the
  SQL surface is genuinely richer (multi-FK `CREATE TABLE`).
- One genuinely new capability — table rename — adds a low-level op
  that the simple mode does not have; it must keep the CSV file name
  and the relationship metadata in step with the table name.
- ADR-0030 §4 is clarified (own `Sql*` DDL commands, structurally
  executed); no behaviour of the shipped DML/`SELECT` phases changes.
- The conversion model unifies simple and advanced without a force
  flag in SQL, relying on `undo` (ADR-0006) as the advanced-mode
  safety net — a concrete payoff of having shipped undo first.

## See also

- **ADR-0030** — the advanced-mode architecture; this is its Phase 4
  and clarifies §4 (DDL representation) and restates §5 (types) / §7
  (neutrality) / §8 (assistance) / §11 (persistence).
- **ADR-0033** — the DML phase; source of the category-grouped
  mode-aware dispatch (Amendment 1) reused for shared entry words.
- **ADR-0031** — `sql_expr`, reused for `CHECK` / `DEFAULT`.
- **ADR-0013** — relationships + the rebuild-table primitive that the
  `ALTER`/FK handlers build on.
- **ADR-0017** — the column type-change classification §7 shares.
- **ADR-0029** — column constraints; **ADR-0025** — indexes;
  **ADR-0011** — FK column-type compatibility; **ADR-0005** — the
  ten-type vocabulary.
- **ADR-0006** — undo; each DDL statement is one undo step (§10).
