Database basics
===============

Yii has a database access layer built on top of PHP's [PDO](http://www.php.net/manual/en/book.pdo.php). It provides
uniform API and solves some inconsistencies between different DBMS. By default Yii supports the following DBMS:

- [MySQL](http://www.mysql.com/)
- [MariaDB](https://mariadb.com/)
- [SQLite](http://sqlite.org/)
- [PostgreSQL](http://www.postgresql.org/)
- [CUBRID](http://www.cubrid.org/): version 9.1.0 or higher.
- [Oracle](http://www.oracle.com/us/products/database/overview/index.html)
- [MSSQL](https://www.microsoft.com/en-us/sqlserver/default.aspx): version 2012 or above is required if you
  want to use LIMIT/OFFSET.


Configuration
-------------

In order to start using database you need to configure database connection component first by adding `db` component
to application configuration (for "basic" web application it's `config/web.php`) like the following:

```php
return [
	// ...
	'components' => [
		// ...
		'db' => [
			'class' => 'yii\db\Connection',
			'dsn' => 'mysql:host=localhost;dbname=mydatabase', // MySQL, MariaDB
			//'dsn' => 'sqlite:/path/to/database/file', // SQLite
			//'dsn' => 'pgsql:host=localhost;port=5432;dbname=mydatabase', // PostgreSQL
			//'dsn' => 'cubrid:dbname=demodb;host=localhost;port=33000', // CUBRID
			//'dsn' => 'sqlsrv:Server=localhost;Database=mydatabase', // MS SQL Server, sqlsrv driver
			//'dsn' => 'dblib:host=localhost;dbname=mydatabase', // MS SQL Server, dblib driver
			//'dsn' => 'mssql:host=localhost;dbname=mydatabase', // MS SQL Server, mssql driver
			//'dsn' => 'oci:dbname=//localhost:1521/mydatabase', // Oracle
			'username' => 'root',
			'password' => '',
			'charset' => 'utf8',
		],
	],
	// ...
];
```

Please refer to the [PHP manual](http://www.php.net/manual/en/function.PDO-construct.php) for more details
on the format of the DSN string.

After the connection component is configured you can access it using the following syntax:

```php
$connection = \Yii::$app->db;
```

You can refer to [[yii\db\Connection]] for a list of properties you can configure. Also note that you can define more
than one connection component and use both at the same time if needed:

```php
$primaryConnection = \Yii::$app->db;
$secondaryConnection = \Yii::$app->secondDb;
```

If you don't want to define the connection as an application component you can instantiate it directly:

```php
$connection = new \yii\db\Connection([
	'dsn' => $dsn,
 	'username' => $username,
 	'password' => $password,
]);
$connection->open();
```


> **Tip**: if you need to execute additional SQL queries right after establishing a connection you can add the
> following to your application configuration file:
>
```php
return [
	// ...
	'components' => [
		// ...
		'db' => [
			'class' => 'yii\db\Connection',
			// ...
			'on afterOpen' => function($event) {
				$event->sender->createCommand("SET time_zone = 'UTC'")->execute();
			}
		],
	],
	// ...
];
```

Basic SQL queries
-----------------

Once you have a connection instance you can execute SQL queries using [[yii\db\Command]].

### SELECT

When query returns a set of rows:

```php
$command = $connection->createCommand('SELECT * FROM tbl_post');
$posts = $command->queryAll();
```

When only a single row is returned:

```php
$command = $connection->createCommand('SELECT * FROM tbl_post WHERE id=1');
$post = $command->queryOne();
```

When there are multiple values from the same column:

```php
$command = $connection->createCommand('SELECT title FROM tbl_post');
$titles = $command->queryColumn();
```

When there's a scalar value:

```php
$command = $connection->createCommand('SELECT COUNT(*) FROM tbl_post');
$postCount = $command->queryScalar();
```

### UPDATE, INSERT, DELETE etc.

If SQL executed doesn't return any data you can use command's `execute` method:

```php
$command = $connection->createCommand('UPDATE tbl_post SET status=1 WHERE id=1');
$command->execute();
```

Alternatively the following syntax that takes care of proper table and column names quoting is possible:

```php
// INSERT
$connection->createCommand()->insert('tbl_user', [
	'name' => 'Sam',
	'age' => 30,
])->execute();

// INSERT multiple rows at once
$connection->createCommand()->batchInsert('tbl_user', ['name', 'age'], [
	['Tom', 30],
	['Jane', 20],
	['Linda', 25],
])->execute();

// UPDATE
$connection->createCommand()->update('tbl_user', ['status' => 1], 'age > 30')->execute();

// DELETE
$connection->createCommand()->delete('tbl_user', 'status = 0')->execute();
```

Quoting table and column names
------------------------------

Most of the time you would use the following syntax for quoting table and column names:

```php
$sql = "SELECT COUNT([[$column]]) FROM {{$table}}";
$rowCount = $connection->createCommand($sql)->queryScalar();
```

In the code above `[[X]]` will be converted to properly quoted column name while `{{Y}}` will be converted to properly
quoted table name.

The alternative is to quote table and column names manually using [[yii\db\Connection::quoteTableName()]] and
[[yii\db\Connection::quoteColumnName()]]:

```php
$column = $connection->quoteColumnName($column);
$table = $connection->quoteTableName($table);
$sql = "SELECT COUNT($column) FROM $table";
$rowCount = $connection->createCommand($sql)->queryScalar();
```

Prepared statements
-------------------

In order to securely pass query parameters you can use prepared statements:

```php
$command = $connection->createCommand('SELECT * FROM tbl_post WHERE id=:id');
$command->bindValue(':id', $_GET['id']);
$post = $command->query();
```

Another usage is performing a query multiple times while preparing it only once:

```php
$command = $connection->createCommand('DELETE FROM tbl_post WHERE id=:id');
$command->bindParam(':id', $id);

$id = 1;
$command->execute();

$id = 2;
$command->execute();
```

Transactions
------------

You can perform transactional SQL queries like the following:

```php
$transaction = $connection->beginTransaction();
try {
	$connection->createCommand($sql1)->execute();
 	$connection->createCommand($sql2)->execute();
	// ... executing other SQL statements ...
	$transaction->commit();
} catch(Exception $e) {
	$transaction->rollBack();
}
```

You can also nest multiple transactions, if needed:

```php
// outer transaction
$transaction1 = $connection->beginTransaction();
try {
	$connection->createCommand($sql1)->execute();

	// inner transaction
	$transaction2 = $connection->beginTransaction();
	try {
		$connection->createCommand($sql2)->execute();
		$transaction2->commit();
	} catch (Exception $e) {
		$transaction2->rollBack();
	}

	$transaction1->commit();
} catch (Exception $e) {
	$transaction1->rollBack();
}
```


Working with database schema
----------------------------

### Getting schema information

You can get a [[yii\db\Schema]] instance like the following:

```php
$schema = $connection->getSchema();
```

It contains a set of methods allowing you to retrieve various information about the database:

```php
$tables = $schema->getTableNames();
```

For the full reference check [[yii\db\Schema]].

### Modifying schema

Aside from basic SQL queries [[yii\db\Command]] contains a set of methods allowing to modify database schema:

- createTable, renameTable, dropTable, truncateTable
- addColumn, renameColumn, dropColumn, alterColumn
- addPrimaryKey, dropPrimaryKey
- addForeignKey, dropForeignKey
- createIndex, dropIndex

These can be used as follows:

```php
// CREATE TABLE
$connection->createCommand()->createTable('tbl_post', [
	'id' => 'pk',
	'title' => 'string',
	'text' => 'text',
]);
```

For the full reference check [[yii\db\Command]].
