Add Add Check Constraint To Existing Column as a SQLite TIL

sqlite/add-check-constraint-to-existing-column.md

@@ -0,0 +1,74 @@
+# Add Check Constraint To Existing Column
+
+To not bury the lede, SQLite does not support adding a check constraint to an
+existing column. Instead, if I want to add one after the fact, I can reconstruct
+the table with the check constraint specified up front and then migrate the
+data.
+
+Here is a `sessions` table that includes `start_time` and `end_time` columns
+that use the `text` data type to represent points in time.
+
+```sql
+sqlite> .schema sessions
+CREATE TABLE sessions (
+        id integer primary key,
+        active integer not null check (active in (0, 1)),
+        project_id integer not null references projects(id) on delete cascade,
+        start_time text not null,
+        end_time text,
+        created_at text not null default (datetime('now')),
+        updated_at text not null default (datetime('now'))
+    );
+CREATE UNIQUE INDEX idx_sessions_single_active
+        on sessions(active)
+        where active = 1;
+```
+
+I want `start_time` and `end_time` to both enforce the shape of the timestamp
+strings with `check` constraints. To do that, I need to start a transaction,
+create a new version of the table with the check constraints, migrate the data,
+rename `sessions` to `sessions_old`, rename `sessions_new` to `sessions` (that's
+the in-place swap), and then commit the transaction. `sessions_old` can be
+dropped later once I feel good about the migration.
+
+```sql
+begin transaction;
+
+create table sessions_new (
+  id integer primary key,
+  active integer not null check (active in (0, 1)),
+  project_id integer not null references projects(id) on delete cascade,
+  start_time text not null,
+  end_time text,
+  created_at text not null default (datetime('now')),
+  updated_at text not null default (datetime('now')),
+  check(start_time is strftime('%Y-%m-%dT%H:%M:%fZ', start_time)),
+  check(end_time is strftime('%Y-%m-%dT%H:%M:%fZ', end_time))
+);
+
+insert into sessions_new (
+  id, active, project_id, start_time, end_time, created_at, updated_at
+)
+select id, active, project_id, start_time, end_time, created_at, updated_at
+from sessions;
+
+alter table sessions rename to sessions_old;
+
+alter table sessions_new rename to sessions;
+
+drop index if exists idx_sessions_single_active; -- on sessions_old
+
+create unique index idx_sessions_single_active
+  on sessions(active)
+  where active = 1;
+
+commit;
+```
+
+Notice that after the table renames I also drop index (now pointing to
+`sessions_old`) and recreate it for the _new_ `sessions` table.
+
+This approach worked well for my situation, but may not be a one-size-fits-all
+solution. Depending on how the database is deployed, the size of the tables, and
+usage, this approach may not scale. Always do a dry-run of database migrations
+like this.