# DbDependency

> DbDependency represents a dependency based on the query result of a SQL statement.

[API reference](/docs/api/index.md) › [yii](/docs/api/yii/index.md) › [caching](/docs/api/yii/caching/index.md) › DbDependency

| class | `yii\caching\DbDependency` |
| --- | --- |
| Extends | [`Dependency`](/docs/api/yii/caching/dependency/index.md) » [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| Implements | [`yii\base\Configurable`](/docs/api/yii/base/configurable/index.md) |
| Since | 2.0 |
| Source | [`framework/caching/DbDependency.php`](https://github.com/yiisoft/yii2/blob/22.0/framework/caching/DbDependency.php#L26) |

DbDependency represents a dependency based on the query result of a SQL statement.

If the query result changes, the dependency is considered as changed.
The query is specified via the [`$sql`](/docs/api/yii/caching/dbdependency/index.md#property-sql) property.

For more details and usage information on Cache, see the [guide article on caching](/docs/guide/caching-overview/index.md).

## Properties

| Property | Type | Description | Defined by |
| --- | --- | --- | --- |
| <span id="property-data"></span>[`$data`](/docs/api/yii/caching/dependency/index.md#property-data) | `mixed` | The dependency data that is saved in cache and later is compared with the latest dependency data. Default: `null`. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |
| <span id="property-db"></span>`$db` | [`yii\db\Connection`](/docs/api/yii/db/connection/index.md)`\|string` | The DB connection object or the application component ID of the DB connection. Default: `'db'`. | `DbDependency` |
| <span id="property-params"></span>`$params` | `array` | The parameters (name => value) to be bound to the SQL statement specified by [`$sql`](/docs/api/yii/caching/dbdependency/index.md#property-sql). Default: `[]`. | `DbDependency` |
| <span id="property-reusable"></span>[`$reusable`](/docs/api/yii/caching/dependency/index.md#property-reusable) | `bool` | Whether this dependency is reusable or not. Default: `false`. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |
| [`$sql`](#property-sql) | `string\|null` | The SQL query whose result is used to determine if the dependency has been changed. Default: `null`. | `DbDependency` |

## Methods

| Method | Description | Defined by |
| --- | --- | --- |
| <span id="method-__construct"></span>[`__construct()`](/docs/api/yii/base/baseobject/index.md#method-__construct) | Constructor. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-__call"></span>[`__call()`](/docs/api/yii/base/baseobject/index.md#method-__call) | Calls the named method which is not a class method. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-__get"></span>[`__get()`](/docs/api/yii/base/baseobject/index.md#method-__get) | Returns the value of an object property. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-__isset"></span>[`__isset()`](/docs/api/yii/base/baseobject/index.md#method-__isset) | Checks if a property is set, i.e. defined and not null. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-__set"></span>[`__set()`](/docs/api/yii/base/baseobject/index.md#method-__set) | Sets value of an object property. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-__unset"></span>[`__unset()`](/docs/api/yii/base/baseobject/index.md#method-__unset) | Sets an object property to null. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-canGetProperty"></span>[`canGetProperty()`](/docs/api/yii/base/baseobject/index.md#method-canGetProperty) | Returns a value indicating whether a property can be read. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-canSetProperty"></span>[`canSetProperty()`](/docs/api/yii/base/baseobject/index.md#method-canSetProperty) | Returns a value indicating whether a property can be set. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-className"></span>[`className()`](/docs/api/yii/base/baseobject/index.md#method-className) static | Returns the fully qualified name of this class. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-evaluateDependency"></span>[`evaluateDependency()`](/docs/api/yii/caching/dependency/index.md#method-evaluateDependency) | Evaluates the dependency by generating and saving the data related with dependency. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |
| <span id="method-hasMethod"></span>[`hasMethod()`](/docs/api/yii/base/baseobject/index.md#method-hasMethod) | Returns a value indicating whether a method is defined. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-hasProperty"></span>[`hasProperty()`](/docs/api/yii/base/baseobject/index.md#method-hasProperty) | Returns a value indicating whether a property is defined. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-init"></span>[`init()`](/docs/api/yii/base/baseobject/index.md#method-init) | Initializes the object. | [`yii\base\BaseObject`](/docs/api/yii/base/baseobject/index.md) |
| <span id="method-isChanged"></span>[`isChanged()`](/docs/api/yii/caching/dependency/index.md#method-isChanged) | Checks whether the dependency is changed. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |
| <span id="method-resetReusableData"></span>[`resetReusableData()`](/docs/api/yii/caching/dependency/index.md#method-resetReusableData) static | Resets all cached data for reusable dependencies. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |
| [`generateDependencyData()`](#method-generateDependencyData) protected | Generates the data needed to determine if dependency has been changed. | `DbDependency` |
| <span id="method-generateReusableHash"></span>[`generateReusableHash()`](/docs/api/yii/caching/dependency/index.md#method-generateReusableHash) protected | Generates a unique hash that can be used for retrieving reusable dependency data. | [`Dependency`](/docs/api/yii/caching/dependency/index.md) |

## Property details

### $sql <span id="property-sql"></span>

```php
public string|null $sql = null
```

The SQL query whose result is used to determine if the dependency has been changed.

Only the first row of the query result will be used.

## Method details

### generateDependencyData() <span id="method-generateDependencyData"></span>

```php
protected function generateDependencyData(yii\caching\CacheInterface $cache): mixed
```

Generates the data needed to determine if dependency has been changed.

This method returns the value of the global state.

**Parameters**

| Parameter | Type | Description |
| --- | --- | --- |
| `$cache` | [`CacheInterface`](/docs/api/yii/caching/cacheinterface/index.md) | The cache component that is currently evaluating this dependency |

**Returns**

`mixed`: The data needed to determine if dependency has been changed.

**Throws**

- [`yii\base\InvalidConfigException`](/docs/api/yii/base/invalidconfigexception/index.md): if [`$db`](/docs/api/yii/caching/dbdependency/index.md#property-db) is not a valid application component ID