# MySQL to DSQL: Structural Changes

Part of [MySQL to DSQL DDL Migration](ddl-operations.md). See [Common Verify & Swap Pattern](ddl-operations.md#common-verify--swap-pattern) for the shared migration end-pattern.

---

## ADD/DROP CONSTRAINT Migration

**MySQL syntax:**

```sql
ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);
ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK (condition);
ALTER TABLE table_name DROP CONSTRAINT constraint_name;
-- or MySQL-specific:
ALTER TABLE table_name DROP INDEX index_name;
ALTER TABLE table_name DROP CHECK constraint_name;
```

**DSQL:** MUST use **Table Recreation Pattern**.

### Pre-Migration Validation (for ADD CONSTRAINT)

**MUST validate existing data satisfies the new constraint.**

```sql
-- For UNIQUE constraint: check for duplicates
readonly_query(
  "SELECT target_column, COUNT(*) as cnt FROM target_table
   GROUP BY target_column HAVING COUNT(*) > 1 LIMIT 10"
)
-- MUST ABORT if any duplicates exist

-- For CHECK constraint: validate all rows pass
readonly_query(
  "SELECT COUNT(*) as invalid_count FROM target_table
   WHERE NOT (check_condition)"
)
-- MUST ABORT if invalid_count > 0
```

### Migration Steps (ADD CONSTRAINT)

#### Step 1: Create new table with the constraint

```sql
transact([
  "CREATE TABLE target_table_new (
     id UUID PRIMARY KEY,
     email VARCHAR(255) UNIQUE,  -- Added UNIQUE constraint
     age INTEGER CHECK (age >= 0),  -- Added CHECK constraint
     other_column TEXT
   )"
])
```

#### Step 2: Copy data

```sql
transact([
  "INSERT INTO target_table_new (id, email, age, other_column)
   SELECT id, email, age, other_column
   FROM target_table"
])
```

**Step 3: Verify and swap** (see [Common Pattern](ddl-operations.md#common-verify--swap-pattern))

### Migration Steps (DROP CONSTRAINT)

#### Step 1: Identify existing constraints

```sql
readonly_query(
  "SELECT constraint_name, constraint_type
   FROM information_schema.table_constraints
   WHERE table_name = 'target_table'
   AND constraint_type IN ('UNIQUE', 'CHECK')"
)
```

#### Step 2: Create new table without the constraint

```sql
transact([
  "CREATE TABLE target_table_new (
     id UUID PRIMARY KEY,
     email VARCHAR(255),  -- Removed UNIQUE constraint
     other_column TEXT
   )"
])
```

#### Step 3: Copy data

```sql
transact([
  "INSERT INTO target_table_new (id, email, other_column)
   SELECT id, email, other_column
   FROM target_table"
])
```

**Step 4: Verify and swap** (see [Common Pattern](ddl-operations.md#common-verify--swap-pattern))

---

## MODIFY PRIMARY KEY Migration

**MySQL syntax:**

```sql
ALTER TABLE table_name DROP PRIMARY KEY, ADD PRIMARY KEY (new_column);
```

**DSQL:** MUST use **Table Recreation Pattern**.

### Pre-Migration Validation

**MUST validate new PK column has unique, non-null values.**

```sql
-- Check for duplicates
readonly_query(
  "SELECT new_pk_column, COUNT(*) as cnt FROM target_table
   GROUP BY new_pk_column HAVING COUNT(*) > 1 LIMIT 10"
)
-- MUST ABORT if any duplicates exist

-- Check for NULLs
readonly_query(
  "SELECT COUNT(*) as null_count FROM target_table
   WHERE new_pk_column IS NULL"
)
-- MUST ABORT if null_count > 0
```

### Migration Steps

#### Step 1: Create new table with new primary key

```sql
transact([
  "CREATE TABLE target_table_new (
     new_pk_column UUID PRIMARY KEY,  -- New PK
     old_pk_column VARCHAR(255),      -- Demoted to regular column
     other_column TEXT
   )"
])
```

#### Step 2: Copy data

```sql
transact([
  "INSERT INTO target_table_new (new_pk_column, old_pk_column, other_column)
   SELECT new_pk_column, old_pk_column, other_column
   FROM target_table"
])
```

**Step 3: Verify and swap** (see [Common Pattern](ddl-operations.md#common-verify--swap-pattern))