# Creating Table Constraints

Sometimes you want to add constraints on a table level, rather than a column level. The following methods will let you accomplish that.

## index

Create a generic index from one or more columns.

| Argument | Type            | Required | Default                                                           | Description                                            |
| -------- | --------------- | -------- | ----------------------------------------------------------------- | ------------------------------------------------------ |
| columns  | string or array | `true`   |                                                                   | The column or array of columns that make up the index. |
| name     | string          | `false`  | A generated name consisting of the table name and column name(s). | The name of the index constraint.                      |
|          |                 |          |                                                                   |                                                        |

**Example:**

**SchemaBuilder**

```
schema.create( "users", function( table ) {
    table.string( "first_name" );
    table.string( "last_name" );
    table.index( [ "first_name", "last_name" ], "idx_users_full_name" );
} );
```

**SQL (MySQL)**

```sql
CREATE TABLE `users` (
    `first_name` VARCHAR(255) NOT NULL,
    `last_name` VARCHAR(255) NOT NULL,
    INDEX `idx_users_full_name` (`first_name`, `last_name`)
)
```

## foreignKey

Create a foreign key constraint from one or more columns. Follow up this call with calls to the `TableIndex`'s [`references`](https://qb.ortusbooks.com/8.3.0/schema-builder/broken-reference) and [`onTable`](https://qb.ortusbooks.com/8.3.0/schema-builder/broken-reference) methods.

| Argument | Type            | Required | Default                                                           | Description                                                                    |
| -------- | --------------- | -------- | ----------------------------------------------------------------- | ------------------------------------------------------------------------------ |
| columns  | string or array | `true`   |                                                                   | The column or array of columns that references a key or keys on another table. |
| name     | string          | `false`  | A generated name consisting of the table name and column name(s). | The name of the foreign key constraint.                                        |

**Example:**

**SchemaBuilder**

```
schema.create( "users", function( table ) {
    table.unsignedInteger( "country_id" );
    table.foreignKey( "country_id" ).references( "id" ).onTable( "countries" );
} );
```

**SQL (MySQL)**

```sql
CREATE TABLE `users` (
    `country_id` INTEGER UNSIGNED NOT NULL,
    CONSTRAINT `fk_users_country_id` FOREIGN KEY (`country_id`) REFERENCES `countries` (`id`) ON UPDATE NO ACTION ON DELETE NO ACTION
)
```

## primaryKey

Create a primary key constraint from one or more columns.

| Argument | Type            | Required | Default                                                           | Description                                                  |
| -------- | --------------- | -------- | ----------------------------------------------------------------- | ------------------------------------------------------------ |
| columns  | string or array | `true`   |                                                                   | The column or array of columns that make up the primary key. |
| name     | string          | `false`  | A generated name consisting of the table name and column name(s). | The name of the primary key constraint.                      |
|          |                 |          |                                                                   |                                                              |

**Example:**

**SchemaBuilder**

```
schema.create( "posts_users", function( table ) {
    table.unsignedInteger( "post_id" ).references( "id" ).onTable( "posts" );
    table.unsignedInteger( "user_id" ).references( "id" ).onTable( "users" );
    table.primaryKey( [ "post_id", "user_id" ], "pk_posts_users" );
} );
```

**SQL (MySQL)**

```sql
CREATE TABLE `posts_users` (
    `post_id` VARCHAR(255) NOT NULL,
    `user_id` VARCHAR(255) NOT NULL,
    INDEX `idx_users_full_name` (`first_name`, `last_name`),
    CONSTRAINT `fk_posts_users_post_id` FOREIGN KEY (`post_id`) REFERENCES `posts` (`id`) ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT `fk_posts_users_user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT ""pk_users_first_name_last_name"" PRIMARY KEY (""first_name"", ""last_name"")
)
```

## unique

Create a unique constraint from one or more columns.

| Argument | Type            | Required | Default                                                           | Description                                                        |
| -------- | --------------- | -------- | ----------------------------------------------------------------- | ------------------------------------------------------------------ |
| columns  | string or array | `true`   |                                                                   | The column or array of columns that make up the unique constraint. |
| name     | string          | `false`  | A generated name consisting of the table name and column name(s). | The name of the unique constraint.                                 |

**Example:**

**SchemaBuilder**

```
schema.create( "users", function( table ) {
    table.increments( "id" );
    table.string( "username ");
    table.unique( "username" );
} );
```

**SQL (MySQL)**

```sql
CREATE TABLE `users` (
    `id` INTEGER UNSIGNED NOT NULL AUTO_INCREMENT,
    UNIQUE (`username`)
)
```