### boundary SQL checks
# A fresh test table was used for the existing-versus-missing constraint boundary.

```sql
ALTER TABLE users DROP CONSTRAINT IF EXISTS missing_boundary;
```

### IF EXISTS response
ERROR: 42601: at or near "if": syntax error

### catalog readback
boundary_email_key
boundary_note_idx

### comparison response without IF EXISTS
```sql
ALTER TABLE users DROP CONSTRAINT missing_boundary;
```

ERROR: 42704: Constraint "missing_boundary" does not exist

### uniqueness readback
```sql
INSERT INTO users (email) VALUES ('dup@example.com');
```

ERROR: 23505: duplicate unique key given: [dup@example.com]

# final result: The missing-constraint IF EXISTS form returned syntax error 42601 instead of a no-op. The existing unique constraint and unrelated index remained present, and duplicate insertion was rejected with 23505.