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Database basics
===============
Yii has a database access layer built on top of PHP's [PDO](http://www.php.net/manual/en/ref.pdo.php). It provides
uniform API and solves some inconsistencies between different DBMS. By default Yii supports MySQL, SQLite, PostgreSQL,
Oracle and MSSQL.
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 array(
// ...
'components' => array(
// ...
'db' => array(
'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' => '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',
),
),
// ...
);
```
After the 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(array(
'dsn' => $dsn,
'username' => $username,
'password' => $password,
));
$connection->open();
```
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->query();
```
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', array(
'name' => 'Sam',
'age' => 30,
))->execute();
// INSERT multiple rows at once
$connection->createCommand()->batchInsert('tbl_user', array('name', 'age'), array(
array('Tom', 30),
array('Jane', 20),
array('Linda', 25),
))->execute();
// UPDATE
$connection->createCommand()->update('tbl_user', array(
'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
------------
If the underlying DBMS supports 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();
}
```
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', array(
'id' => 'pk',
'title' => 'string',
'text' => 'text',
);
```
For the full reference check [[\yii\db\Command]].