> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cube.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Extending cubes

> Use extends on cubes to inherit and merge members from a parent so shared measures, dimensions, and joins stay defined once.

The `extends` parameter, supported for [cubes][ref-cube-extends] and
[views][ref-view-extends], allows you to create a *child* cube (or a view) that reuses
all declared members of a *parent* cube (or a view). This helps build reusable data models.

Cubes declare members such as measures, dimensions, and segments. When a child cube extends
the parent cube, lists of measures, dimensions, and segments are merged.
For example, if the parent cube defines the `a` measure and the child cube defines the `b`
measure, the resulting cube will have both measures `a` and `b`.

The usual pattern is to extract common measures, dimensions, and joins into
the parent cube and then extend from it. This helps prevent code duplication
and makes code easier to maintain and refactor.

In the example below, the `base_events` cube defines the common events measures,
dimensions, and a join to the `users` cube:

<CodeGroup>
  ```yaml title="YAML" theme={"dark"}
  cubes:
    - name: base_events
      sql_table: events

      joins:
        - name: users
          relationship: many_to_one
          sql: "{CUBE}.user_id = {users.id}"

      measures:
        - name: count
          type: count

      dimensions:
        - name: timestamp
          sql: time
          type: time
  ```

  ```javascript title="JavaScript" theme={"dark"}
  cube(`base_events`, {
    sql_table: `events`,

    joins: {
      users: {
        relationship: `many_to_one`,
        sql: `${CUBE}.user_id = ${users.id}`
      }
    },

    measures: {
      count: {
        type: `count`
      }
    },

    dimensions: {
      timestamp: {
        sql: `time`,
        type: `time`
      }
    }
  })
  ```
</CodeGroup>

<Info>
  It’s important to use the [`CUBE` variable][ref-cube-variable] when referencing members
  and columns of the cube. Not specifying the cube name or using `${base_events}` does not
  work when the cube is extended.
</Info>

The `product_purchases` and `page_views` cubes are extended from `base_events`
and define only the specific dimensions: `product_name` for product purchases
and `page_path` for page views.

<CodeGroup>
  ```yaml title="YAML" theme={"dark"}
  cubes:
    - name: product_purchases
      sql_table: product_purchases
      extends: base_events

      dimensions:
        - name: product_name
          sql: product_name
          type: string

    - name: page_views
      sql_table: page_views
      extends: base_events

      dimensions:
        - name: page_path
          sql: page_path
          type: string
  ```

  ```javascript title="JavaScript" theme={"dark"}
  cube(`product_purchases`, {
    sql_table: `product_purchases`,
    extends: base_events,

    dimensions: {
      product_name: {
        sql: `product_name`,
        type: `string`
      }
    }
  })

  cube(`page_views`, {
    sql_table: `page_views`,
    extends: base_events,

    dimensions: {
      page_path: {
        sql: `page_path`,
        type: `string`
      }
    }
  })
  ```
</CodeGroup>

## Usage with `FILTER_PARAMS`

If the parent cube is using [`FILTER_PARAMS`][ref-schema-ref-cube-filter-params]
in any `sql` parameter, then child cubes can accomodate to that in two ways.

First, the `sql` parameter can be overridden in each child cube:

<CodeGroup>
  ```yaml title="YAML" theme={"dark"}
  cubes:
    - name: product_purchases
      sql: |
        SELECT *
        FROM events
        WHERE {FILTER_PARAMS.product_purchases.timestamp.filter('time')}

      # ...
  ```

  ```javascript title="JavaScript" theme={"dark"}
  cube(`product_purchases`, {
    sql: `
      SELECT *
      FROM events
      WHERE ${FILTER_PARAMS.product_purchases.timestamp.filter("time")}
    `,

    // ...
  })
  ```
</CodeGroup>

Alternatively, all filters can be put inside the parent cube and referenced
in the child cubes using `AND`. The unused filters will be rendered to `1 = 1`
in the SQL query:

<CodeGroup>
  ```yaml title="YAML" theme={"dark"}
  cubes:
    - name: base_events
      sql: |
        SELECT *
        FROM events
        WHERE
          {FILTER_PARAMS.base_events.timestamp.filter('time')} AND
          {FILTER_PARAMS.product_purchases.timestamp.filter('time')} AND
          {FILTER_PARAMS.page_views.timestamp.filter('time')}

      # ...
  ```

  ```javascript title="JavaScript" theme={"dark"}
  cube(`base_events`, {
    sql: `
      SELECT *
      FROM events
      WHERE
        {$FILTER_PARAMS.base_events.timestamp.filter('time')} AND
        {$FILTER_PARAMS.product_purchases.timestamp.filter('time')} AND
        {$FILTER_PARAMS.page_views.timestamp.filter('time')}
    `,

    // ...
  })
  ```
</CodeGroup>

[ref-cube-extends]: /reference/data-modeling/cube#extends

[ref-view-extends]: /reference/data-modeling/view#extends

[ref-schema-ref-cube-filter-params]: /reference/data-modeling/cube#filter-params

[ref-cube-variable]: /docs/data-modeling/syntax#cube-variable