added dabase basics docs

docs/guide/database-basics.md

+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',
+			'username' => 'root',
+			'password' => '',
+			'charset' => 'utf8',
+		),
+	),
+	// ...
+);
+```
+
+After component is configured you can access 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 components 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 connection as 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 = $connection->createCommand('SELECT * FROM tbl_post LIMIT 1');
+$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 = $connection->createCommand('UPDATE tbl_post SET status=1');
+$command->execute();
+```
+
+Alternatively the following syntax 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();
+```
+
+
+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 = $schema->getTableNames();
+```
+
+For the full reference check [[\yii\db\Schema]].
+
+### Modifying schema
+
+Aside from basic SQL queries [[\yii\db\Command]] contains a set of methods allowing to modify database schema:
+
+- createTable, renameTable, dropTable, truncateTable
+- addColumn, renameColumn, dropColumn, alterColumn
+- addPrimaryKey, dropPrimaryKey
+- addForeignKey, dropForeignKey
+- createIndex, dropIndex
+
+These can be used as follows:
+
+```php
+// UPDATE
+$connection->createCommand()->createTable('tbl_post', array(
+	'id' => 'pk',
+	'title' => 'string',
+	'text' => 'text',
+);
+```
+
+For the full reference check [[\yii\db\Command]].

docs/guide/index.md

@@ -23,7 +23,7 @@ Base concepts
 Database
 ========
 
-- [Database Access Objects](dao.md)
+- [Basics](database-basics.md)
 - [Query Builder](query-builder.md)
 - [ActiveRecord](active-record.md)
 - [Database Migration](migration.md)