To connect to a database, first set the database connection parameters in the configuration file. 1. Configure database information in config/db.php or config/web.php, including DSN, username, password, etc.; 2. Use Yii::$app->db to access the configured connection; 3. Write SQL queries or use Active Record to operate data; 4. Create test actions to verify whether the connection is successful; 5. If you need multiple database support, define multiple connections in the configuration and call them separately in the code. Through these steps, the connection and interaction between Yii applications and database can be successfully achieved.

To connect to a database using Yii, you mainly need to configure the database connection in your application's configuration file. This tells Yii how to reach your database and what type of database you're using.

Here's how to do it step by step.

Configure the Database Connection

In a Yii application, database settings are usually defined in the config/db.php file or directly inside the config/web.php (for web applications) or config/console.php (for console apps).

A basic configuration looks like this:

 Return [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=localhost;dbname=your_database_name',
    'username' => 'your_username',
    'password' => 'your_password',
    'charset' => 'utf8',
];

Make sure to replace:

• your_database_name
• your_username
• your_password

If you're not using MySQL, change the DSN accordingly. For example:

• SQLite: 'dsn' => 'sqlite:@app/database/mydatabase.db'
• PostgreSQL: 'dsn' => 'pgsql:host=localhost;dbname=mydb;user=me;password=secret'

This setup is straightforward and works for most small to medium-sized projects.

Use the Database Connection in Your Code

Once configured, Yii automatically sets up the database connection when needed. You can access it anywhere in your code using Yii::$app->db .

For example, to run a simple SQL query:

 $connection = Yii::$app->db;
$command = $connection->createCommand('SELECT * FROM user WHERE id = 1');
$user = $command->queryOne();

You don't have to manually open or close the connection — Yii handles that for you.

Also, if you're using Active Record (which is common in Yii), you don't even need to write raw SQL much of the time. Just define your model classes and use methods like find() , save() , etc.

Test Your Connection

It's always a good idea to test whether your database connection works before diving into complex queries.

You can create a simple controller action like this:

 public function actionTestDb()
{
    try {
        Yii::$app->db->open();
        echo "Database connection is working!";
    } catch (\Exception $e) {
        echo "Failed to connect to the database: " . $e->getMessage();
    }
}

If you see an error message, double-check your DSN, username, and password. Also, make sure your database server is running and accessible from your app.

Common issues include:

• Incorrect host name or port
• Wrong database name
• Missing or wrong credentials
• No remote access permissions (especially on shared hosting)

Optional: Set Up Multiple Database Connections

If your project needs to connect to more than one database (like a read/write split or separate databases for different modules), Yii supports that too.

In your config file, instead of returning a single connection, you define multiple ones:

 Return [
    'components' => [
        'db' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=main_db',
            'username' => 'root',
            'password' => '',
        ],
        'db2' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=secondary_db',
            'username' => 'root',
            'password' => '',
        ],
    ],
];

Then in your code, use them like this:

 // Main database
Yii::$app->db->createCommand(...)->execute();

// Secondary database
Yii::$app->db2->createCommand(...)->execute();

This is helpful in larger applications where data might be spread across different systems.

That's basically it. Connecting to a database in Yii is pretty straightforward once you understand how the configuration works.

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