Data Access Objects (DAO)

Data Access Objects (DAO) provides a generic API to access data stored in different database management systems (DBMS). As a result, the underlying DBMS can be changed to a different one without requiring change of the code which uses DAO to access the data.

Yii DAO is built on top of PHP Data Objects (PDO) which is an extension providing unified data access to many popular DBMS, such as MySQL, PostgreSQL. Therefore, to use Yii DAO, the PDO extension and the specific PDO database driver (e.g. PDO_MYSQL) have to be installed.

Yii DAO mainly consists of the following four classes:

• CDbConnection: represents a connection to a database.
• CDbCommand: represents an SQL statement to execute against a database.
• CDbDataReader: represents a forward-only stream of rows from a query result set.
• CDbTransaction: represents a DB transaction.

In the following, we introduce the usage of Yii DAO in different scenarios.

1. Establishing Database Connection ¶

To establish a database connection, create a CDbConnection instance and activate it. A data source name (DSN) is needed to specify the information required to connect to the database. A username and password may also be needed to establish the connection. An exception will be raised in case an error occurs during establishing the connection (e.g. bad DSN or invalid username/password).

$connection=new CDbConnection($dsn,$username,$password);
// establish connection. You may try...catch possible exceptions
$connection->active=true;
......
$connection->active=false;  // close connection

The format of DSN depends on the PDO database driver in use. In general, a DSN consists of the PDO driver name, followed by a colon, followed by the driver-specific connection syntax. See PDO documentation for complete information. Below is a list of commonly used DSN formats:

• SQLite: sqlite:/path/to/dbfile
• MySQL: mysql:host=localhost;dbname=testdb
• PostgreSQL: pgsql:host=localhost;port=5432;dbname=testdb
• SQL Server: mssql:host=localhost;dbname=testdb
• Oracle: oci:dbname=//localhost:1521/testdb

Because CDbConnection extends from CApplicationComponent, we can also use it as an application component. To do so, configure in a db (or other name) application component in the application configuration as follows,

array(
    ......
    'components'=>array(
        ......
        'db'=>array(
            'class'=>'CDbConnection',
            'connectionString'=>'mysql:host=localhost;dbname=testdb',
            'username'=>'root',
            'password'=>'password',
            'emulatePrepare'=>true,  // needed by some MySQL installations
        ),
    ),
)

We can then access the DB connection via Yii::app()->db which is already activated automatically, unless we explictly configure CDbConnection::autoConnect to be false. Using this approach, the single DB connection can be shared in multiple places in our code.

2. Executing SQL Statements ¶

Once a database connection is established, SQL statements can be executed using CDbCommand. One creates a CDbCommand instance by calling CDbConnection::createCommand() with the specified SQL statement:

$command=$connection->createCommand($sql);
// if needed, the SQL statement may be updated as follows:
// $command->text=$newSQL;

A SQL statement is executed via CDbCommand in one of the following two ways:

• execute(): performs a non-query SQL statement, such as INSERT, UPDATE and DELETE. If successful, it returns the number of rows that are affected by the execution.

• query(): performs an SQL statement that returns rows of data, such as SELECT. If successful, it returns a CDbDataReader instance from which one can traverse the resulting rows of data. For convenience, a set of queryXXX() methods are also implemented which directly return the query results.

An exception will be raised if an error occurs during the execution of SQL statements.

$rowCount=$command->execute();   // execute the non-query SQL
$dataReader=$command->query();   // execute a query SQL
$rows=$command->queryAll();      // query and return all rows of result
$row=$command->queryRow();       // query and return the first row of result
$column=$command->queryColumn(); // query and return the first column of result
$value=$command->queryScalar();  // query and return the first field in the first row

3. Fetching Query Results ¶

After CDbCommand::query() generates the CDbDataReader instance, one can retrieve rows of resulting data by calling CDbDataReader::read() repeatedly. One can also use CDbDataReader in PHP's foreach language construct to retrieve row by row.

$dataReader=$command->query();
// calling read() repeatedly until it returns false
while(($row=$dataReader->read())!==false) { ... }
// using foreach to traverse through every row of data
foreach($dataReader as $row) { ... }
// retrieving all rows at once in a single array
$rows=$dataReader->readAll();

Note: Unlike query(), all queryXXX() methods return data directly. For example, queryRow() returns an array representing the first row of the querying result.

4. Using Transactions ¶

When an application executes a few queries, each reading and/or writing information in the database, it is important to be sure that the database is not left with only some of the queries carried out. A transaction, represented as a CDbTransaction instance in Yii, may be initiated in this case:

• Begin the transaction.
• Execute queries one by one. Any updates to the database are not visible to the outside world.
• Commit the transaction. Updates become visible if the transaction is successful.
• If one of the queries fails, the entire transaction is rolled back.

The above workflow can be implemented using the following code:

$transaction=$connection->beginTransaction();
try
{
    $connection->createCommand($sql1)->execute();
    $connection->createCommand($sql2)->execute();
    //.... other SQL executions
    $transaction->commit();
}
catch(Exception $e) // an exception is raised if a query fails
{
    $transaction->rollBack();
}

5. Binding Parameters ¶

To avoid SQL injection attacks and to improve performance of executing repeatedly used SQL statements, one can "prepare" an SQL statement with optional parameter placeholders that are to be replaced with the actual parameters during the parameter binding process.

The parameter placeholders can be either named (represented as unique tokens) or unnamed (represented as question marks). Call CDbCommand::bindParam() or CDbCommand::bindValue() to replace these placeholders with the actual parameters. The parameters do not need to be quoted: the underlying database driver does it for you. Parameter binding must be done before the SQL statement is executed.

// an SQL with two placeholders ":username" and ":email"
$sql="INSERT INTO users(username, email) VALUES(:username,:email)";
$command=$connection->createCommand($sql);
// replace the placeholder ":username" with the actual username value
$command->bindParam(":username",$username,PDO::PARAM_STR);
// replace the placeholder ":email" with the actual email value
$command->bindParam(":email",$email,PDO::PARAM_STR);
$command->execute();
// insert another row with a new set of parameters
$command->bindParam(":username",$username2,PDO::PARAM_STR);
$command->bindParam(":email",$email2,PDO::PARAM_STR);
$command->execute();

The methods bindParam() and bindValue() are very similar. The only difference is that the former binds a parameter with a PHP variable reference while the latter with a value. For parameters that represent large block of data memory, the former is preferred for performance consideration.

For more details about binding parameters, see the relevant PHP documentation.

6. Binding Columns ¶

When fetching query results, one can also bind columns with PHP variables so that they are automatically populated with the latest data each time a row is fetched.

$sql="SELECT username, email FROM users";
$dataReader=$connection->createCommand($sql)->query();
// bind the 1st column (username) with the $username variable
$dataReader->bindColumn(1,$username);
// bind the 2nd column (email) with the $email variable
$dataReader->bindColumn(2,$email);
while($dataReader->read()!==false)
{
    // $username and $email contain the username and email in the current row
}