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

> Note: This section is under development.

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 2005 or higher.


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',
        ],
    ],
    // ...
];
```

There is a peculiarity when you want to work with the database through the `ODBC` layer. When using `ODBC`,
connection `DSN` doesn't indicate uniquely what database type is being used. That's why you have to override
`driverName` property of [[yii\db\Connection]] class to disambiguate that:

```php
'db' => [
    'class' => 'yii\db\Connection',
    'driverName' => 'mysql',
    'dsn' => 'odbc:Driver={MySQL};Server=localhost;Database=test',
    'username' => 'root',
    'password' => '',
],
```

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 post');
$posts = $command->queryAll();
```

When only a single row is returned:

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

When there are multiple values from the same column:

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

When there's a scalar value:

```php
$command = $connection->createCommand('SELECT COUNT(*) FROM 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 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('user', [
    'name' => 'Sam',
    'age' => 30,
])->execute();

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

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

// DELETE
$connection->createCommand()->delete('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.

For table names there's a special variant `{{%Y}}` that allows you to automatically appending table prefix if it is set:

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

The code above will result in selecting from `tbl_table` if you have table prefix configured like the following in your
config file:

```php
return [
    // ...
    'components' => [
        // ...
        'db' => [
            // ...
            'tablePrefix' => 'tbl_',
        ],
    ],
];
```

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 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 post WHERE id=:id');
$command->bindParam(':id', $id);

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

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

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

When running multiple related queries in a sequence you may need to wrap them in a transaction to
ensure you data is consistent. Yii provides a simple interface to work with transactions in simple
cases but also for advanced usage when you need to define isolation levels.

The following code shows a simple pattern that all code that uses transactional queries should follow:

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

The first line starts a new transaction using the [[yii\db\Connection::beginTransaction()|beginTransaction()]]-method of the database connection
object. The transaction itself is represented by a [[yii\db\Transaction]] object stored in `$transaction`.
We wrap the execution of all queries in a try-catch-block to be able to handle errors.
We call [[yii\db\Transaction::commit()|commit()]] on success to commit the transaction and
[[yii\db\Transaction::rollBack()|rollBack()]] in case of an error. This will revert the effect of all queries
that have been executed inside of the transaction.
`throw $e` is used to re-throw the exception in case we can not handle the error ourselves and delegate it
to some other code or the yii error handler.

It is also possible to 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();
}
```

Note that your DBMS should have support for Savepoints for this to work as expected.
The above code will work for any DBMS but transactional safety is only guaranteed if
the underlying DBMS supports it.

Yii also supports setting [isolation levels] for your transactions.
When beginning a transaction it will run in the default isolation level set by you database system.
You can specifying an isolation level explicitly when starting a transaction:

```php
$transaction = $connection->beginTransaction(\yii\db\Transaction::REPEATABLE_READ);
```

Yii provides four constants for the most common isolation levels:

- [[\yii\db\Transaction::READ_UNCOMMITTED]] - the weakest level, Dirty reads, Non-repeatable reads and Phantoms may occur.
- [[\yii\db\Transaction::READ_COMMITTED]] - avoid Dirty reads.
- [[\yii\db\Transaction::REPEATABLE_READ]] - avoid Dirty reads and Non-repeatable reads.
- [[\yii\db\Transaction::SERIALIZABLE]] - the strongest level, avoids all of the above named problems.

You may use the constants named above but you can also use a string that represents a valid syntax that can be
used in your DBMS following `SET TRANSACTION ISOLATION LEVEL`. For postgres this could be for example
`SERIALIZABLE READ ONLY DEFERRABLE`.

Note that some DBMS allow setting of the isolation level only for the whole connection so subsequent transactions
may get the same isolation level even if you did not specify any. When using this feature
you may need to set the isolation level for all transactions explicitly to avoid conflicting settings.
At the time of this writing affected DBMS are MSSQL and SQLite.

> Note: SQLite only supports two isolation levels, so you can only use `READ UNCOMMITTED` and `SERIALIZABLE`.
Usage of other levels will result in an exception to be thrown.

> Note: PostgreSQL does not allow setting the isolation level before the transaction starts so you can not
specify the isolation level directly when starting the transaction.
You have to call [[yii\db\Transaction::setIsolationLevel()]] in this case after the transaction has started.

[isolation levels]: http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#Isolation_levels


Replication and Read-Write Splitting
------------------------------------

Many DBMS support [database replication](http://en.wikipedia.org/wiki/Replication_(computing)#Database_replication)
to get better database availability and faster server response time. With database replication, data are replicated
from the so-called *master servers* to *slave servers*. All writes and updates must take place on the master servers,
while reads may take place on the slave servers.

To take advantage of database replication and achieve read-write splitting, you can configure a [[yii\db\Connection]]
component like the following:

```php
[
    'class' => 'yii\db\Connection',

    // configuration for the master
    'dsn' => 'dsn for master server',
    'username' => 'master',
    'password' => '',

    // common configuration for slaves
    'slaveConfig' => [
        'username' => 'slave',
        'password' => '',
        'attributes' => [
            // use a smaller connection timeout
            PDO::ATTR_TIMEOUT => 10,
        ],
    ],

    // list of slave configurations
    'slaves' => [
        ['dsn' => 'dsn for slave server 1'],
        ['dsn' => 'dsn for slave server 2'],
        ['dsn' => 'dsn for slave server 3'],
        ['dsn' => 'dsn for slave server 4'],
    ],
]
```

The above configuration specifies a setup with a single master and multiple slaves. One of the slaves will
be connected and used to perform read queries, while the master will be used to perform write queries.
Such read-write splitting is accomplished automatically with this configuration. For example,

```php
// create a Connection instance using the above configuration
$db = Yii::createObject($config);

// query against one of the slaves
$rows = $db->createCommand('SELECT * FROM user LIMIT 10')->queryAll();

// query against the master
$db->createCommand("UPDATE user SET username='demo' WHERE id=1")->execute();
```

> Info: Queries performed by calling [[yii\db\Command::execute()]] are considered as write queries, while
  all other queries done through one of the "query" method of [[yii\db\Command]] are read queries.
  You can get the currently active slave connection via `$db->slave`.

The `Connection` component supports load balancing and failover about slaves.
When performing a read query for the first time, the `Connection` component will randomly pick a slave and
try connecting to it. If the slave is found "dead", it will try another one. If none of the slaves is available,
it will connect to the master. By configuring a [[yii\db\Connection::serverStatusCache|server status cache]],
a "dead" server can be remembered so that it will not be tried again during a
[[yii\db\Connection::serverRetryInterval|certain period of time]].

> Info: In the above configuration, a connection timeout of 10 seconds is specified for every slave.
  This means if a slave cannot be reached in 10 seconds, it is considered as "dead". You can adjust this parameter
  based on your actual environment.


You can also configure multiple masters with multiple slaves. For example,


```php
[
    'class' => 'yii\db\Connection',

    // common configuration for masters
    'masterConfig' => [
        'username' => 'master',
        'password' => '',
        'attributes' => [
            // use a smaller connection timeout
            PDO::ATTR_TIMEOUT => 10,
        ],
    ],

    // list of master configurations
    'masters' => [
        ['dsn' => 'dsn for master server 1'],
        ['dsn' => 'dsn for master server 2'],
    ],

    // common configuration for slaves
    'slaveConfig' => [
        'username' => 'slave',
        'password' => '',
        'attributes' => [
            // use a smaller connection timeout
            PDO::ATTR_TIMEOUT => 10,
        ],
    ],

    // list of slave configurations
    'slaves' => [
        ['dsn' => 'dsn for slave server 1'],
        ['dsn' => 'dsn for slave server 2'],
        ['dsn' => 'dsn for slave server 3'],
        ['dsn' => 'dsn for slave server 4'],
    ],
]
```

The above configuration specifies two masters and four slaves. The `Connection` component also supports
load balancing and failover about masters, like that about slaves. A difference is that in case none of
the masters is available, an exception will be thrown.

> Note: When you use the [[yii\db\Connection::masters|masters]] property to configure one or multiple
  masters, all other properties for specifying a database connection (e.g. `dsn`, `username`, `password`)
  with the `Connection` object itself will be ignored.


By default, transactions use the master connection. And within a transaction, all DB operations will use
the master connection. For example,

```php
// the transaction is started on the master connection
$transaction = $db->beginTransaction();

try {
    // both queries are performed against the master
    $rows = $db->createCommand('SELECT * FROM user LIMIT 10')->queryAll();
    $db->createCommand("UPDATE user SET username='demo' WHERE id=1")->execute();

    $transaction->commit();
} catch(\Exception $e) {
    $transaction->rollBack();
    throw $e;
}
```

If you want to start a transaction with the slave connection, you should explicitly do so, like the following:

```php
$transaction = $db->slave->beginTransaction();
```

Sometimes, you may want to force using the master connection to perform a read query. This can be achieved
with the `useMaster()` method:

```php
$rows = $db->useMaster(function ($db) {
    return $db->createCommand('SELECT * FROM user LIMIT 10')->queryAll();
});
```

You may also directly set `$db->enableSlaves` to be false to direct all queries to the master connection.


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('post', [
    'id' => 'pk',
    'title' => 'string',
    'text' => 'text',
]);
```

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