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Calculating nested aggregates

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Use case

Sometimes, there’s a need to calculate a double aggregation over a fact table. For example, if you have a line_items table that has store_id, order_id, and sales columns, you might wonder what is the median of sales per product for each store. With an ad-hoc SQL query, this double aggregation would probably be expressed as follows: 
WITH sales_per_store_product AS (
  SELECT store_id, product_id, SUM(sales) AS sales
  FROM line_items
  GROUP BY 1, 2
)

SELECT store_id, PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY sales) AS sales_median
FROM sales_per_store_product
GROUP BY 1

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Data modeling

In Cube, measures are used to define aggregates. However, a single measure can only contain a single aggregation, e.g., SUM, APPROX_COUNT_DISTINCT, or PERCENTILE_CONT. If you’d like to define a double aggregation, e.g., a median of a sum of values, the outer aggregation would need to be defined in a separate cube and the inner aggregation (measure) would need to be brought to that cube as a subquery dimension. Also, these cubes would need to have a join definition between them. Consider the following data model: 
cubes:
  - name: nested_agg_sales
    sql: |
      SELECT 1 AS id, 1 AS store_id, 1 AS product_id, 10 AS sales UNION ALL
      SELECT 2 AS id, 1 AS store_id, 1 AS product_id, 20 AS sales UNION ALL
      SELECT 3 AS id, 1 AS store_id, 2 AS product_id, 30 AS sales UNION ALL
      SELECT 4 AS id, 1 AS store_id, 2 AS product_id, 40 AS sales UNION ALL
      SELECT 5 AS id, 2 AS store_id, 1 AS product_id, 50 AS sales UNION ALL
      SELECT 6 AS id, 2 AS store_id, 1 AS product_id, 60 AS sales UNION ALL
      SELECT 7 AS id, 2 AS store_id, 2 AS product_id, 70 AS sales UNION ALL
      SELECT 8 AS id, 2 AS store_id, 2 AS product_id, 80 AS sales

    dimensions:
      - name: id
        sql: id
        type: number
        primary_key: true

      - name: store_id
        sql: store_id
        type: number

      - name: product_id
        sql: product_id
        type: number

      - name: store_product_id
        sql: "CONCAT({store_id}, '-', {product_id})"
        type: string

    measures:
      - name: sales
        sql: sales
        type: sum

  - name: nested_agg_stores_orders
    sql: |
      SELECT store_id, product_id
      FROM (
        SELECT 1 AS id, 1 AS store_id, 1 AS product_id, 10 AS sales UNION ALL
        SELECT 2 AS id, 1 AS store_id, 1 AS product_id, 20 AS sales UNION ALL
        SELECT 3 AS id, 1 AS store_id, 2 AS product_id, 30 AS sales UNION ALL
        SELECT 4 AS id, 1 AS store_id, 2 AS product_id, 40 AS sales UNION ALL
        SELECT 5 AS id, 2 AS store_id, 1 AS product_id, 50 AS sales UNION ALL
        SELECT 6 AS id, 2 AS store_id, 1 AS product_id, 60 AS sales UNION ALL
        SELECT 7 AS id, 2 AS store_id, 2 AS product_id, 70 AS sales UNION ALL
        SELECT 8 AS id, 2 AS store_id, 2 AS product_id, 80 AS sales
      ) AS raw
      GROUP BY 1, 2

    joins:
      - name: nested_agg_sales
        sql: "{nested_agg_stores_orders.store_product_id} = {nested_agg_sales.store_product_id}"
        relationship: one_to_many

    dimensions:
      - name: store_id
        sql: store_id
        type: number

      - name: product_id
        sql: product_id
        type: number

      - name: store_product_id
        sql: "CONCAT({store_id}, '-', {product_id})"
        type: string
        primary_key: true

      - name: sales_sum
        sql: "{nested_agg_sales.sales}"
        type: number
        sub_query: true

    measures:
      - name: median_sales
        sql: "PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY {sales_sum})"
        type: number
As you can see, the sum of sales for per store and per product is defined in the nested_agg_sales cube as the sales measure. Then, it is brought to the nested_agg_stores_orders cube as sales_sum that is defined as a subquery dimension. Also, a join is defined between both cubes. Then, the median of sales is defined as the median_sales measure in the nested_agg_stores_orders cube. It’s OK to reference sales_sum in this measure because now it’s a dimension; referencing a measure from another cube here would not work.

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Result

Querying the median_sales measure would give the expected result:
We can verify that it’s correct by adding one more dimension to the query: