# 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`](broken://pages/-LA-U_axqzow_9OdzTRT#references) and [`onTable`](broken://pages/-LA-U_axqzow_9OdzTRT#onTable) 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`)
)
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://qb.ortusbooks.com/schema-builder/creating-table-constraints.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.