ROLLUP, CUBE, GROUPING Functions and GROUPING SETS in ORACLE

ROLLUP, CUBE, GROUPING Functions and GROUPING SETS

This article gives an overview of the functionality available for aggregation in data warehouses, focusing specifically on the information required for the Oracle Database SQL Expert (1Z0-047) exam.

• Setup
• GROUP BY
• ROLLUP
• CUBE
• GROUPING Functions
  
  • GROUPING Function
  • GROUPING_ID Function
  • GROUP_ID Function
• GROUPING SETS
• Composite Columns
• Concatenated Groupings

GROUP BY

Let's start be reminding ourselves how the GROUP BY clause works. An aggregate function takes multiple rows of data returned by a query and aggregates them into a single result row.

SELECT fact_1_id,
       fact_2_id,
       COUNT(*) AS num_rows,
       SUM(sales_value) AS sales_value
FROM   dimension_tab
GROUP BY fact_1_id, fact_2_id
ORDER BY fact_1_id, fact_2_id;

ROLLUP

In addition to the regular aggregation results we expect from the GROUP BY clause, the ROLLUP extension produces group subtotals from right to left and a grand total. If "n" is the number of columns listed in the ROLLUP, there will be n+1 levels of subtotals.

SELECT fact_1_id,
       fact_2_id,
       SUM(sales_value) AS sales_value
FROM   dimension_tab
GROUP BY ROLLUP (fact_1_id, fact_2_id)
ORDER BY fact_1_id, fact_2_id;

CUBE

In addition to the subtotals generated by the ROLLUP extension, the CUBE extension will generate subtotals for all combinations of the dimensions specified. If "n" is the number of columns listed in the CUBE, there will be 2ⁿ subtotal combinations.
SELECT fact_1_id, fact_2_id, SUM(sales_value) AS sales_value FROM dimension_tab GROUP BY CUBE (fact_1_id, fact_2_id) ORDER BY fact_1_id, fact_2_id;

GROUPING Functions

GROUPING

It can be quite easy to visually identify subtotals generated by rollups and cubes, but to do it programatically you really need something more accurate than the presence of null values in the grouping columns. This is where the GROUPING function comes in. It accepts a single column as a parameter and returns "1" if the column contains a null value generated as part of a subtotal by a ROLLUP or CUBE operation or "0" for any other value, including stored null values.
The following query is a repeat of a previous cube, but the GROUPING function has been added for each of the dimensions in the cube.

SELECT fact_1_id,
       fact_2_id,
       SUM(sales_value) AS sales_value,
       GROUPING(fact_1_id) AS f1g, 
       GROUPING(fact_2_id) AS f2g
FROM   dimension_tab
GROUP BY CUBE (fact_1_id, fact_2_id)
ORDER BY fact_1_id, fact_2_id;

GROUPING_ID

The GROUPING_ID function provides an alternate and more compact way to identify subtotal rows. Passing the dimension columns as arguments, it returns a number indicating the GROUP BY level.
SELECT fact_1_id, fact_2_id, SUM(sales_value) AS sales_value, GROUPING_ID(fact_1_id, fact_2_id) AS grouping_id FROM dimension_tab GROUP BY CUBE (fact_1_id, fact_2_id) ORDER BY fact_1_id, fact_2_id;

GROUP_ID

It's possible to write queries that return the duplicate subtotals, which can be a little confusing. The GROUP_ID function assigns the value "0" to the first set, and all subsequent sets get assigned a higher number. The following query forces duplicates to show the GROUP_ID function in action.

SELECT fact_1_id,
       fact_2_id,
       SUM(sales_value) AS sales_value,
       GROUPING_ID(fact_1_id, fact_2_id) AS grouping_id,
       GROUP_ID() AS group_id
FROM   dimension_tab
GROUP BY GROUPING SETS(fact_1_id, CUBE (fact_1_id, fact_2_id))
ORDER BY fact_1_id, fact_2_id

GROUPING SETS

Calculating all possible subtotals in a cube, especially those with many dimensions, can be quite an intensive process. If you don't need all the subtotals, this can represent a considerable amount of wasted effort. The following cube with three dimensions gives 8 levels of subtotals (GROUPING_ID: 0-7), shown here.

SELECT fact_1_id,
       fact_2_id,
       fact_3_id,
       SUM(sales_value) AS sales_value,
       GROUPING_ID(fact_1_id, fact_2_id, fact_3_id) AS grouping_id
FROM   dimension_tab
GROUP BY CUBE(fact_1_id, fact_2_id, fact_3_id)
ORDER BY fact_1_id, fact_2_id, fact_3_id;

If we only need a few of these levels of subtotaling we can use the GROUPING SETS expression and specify exactly which ones we need, saving us having to calculate the whole cube. In the following query we are only interested in subtotals for the "FACT_1_ID, FACT_2_ID" and "FACT_1_ID, FACT_3_ID" groups.

SELECT fact_1_id,
       fact_2_id,
       fact_3_id,
       SUM(sales_value) AS sales_value,
       GROUPING_ID(fact_1_id, fact_2_id, fact_3_id) AS grouping_id
FROM   dimension_tab
GROUP BY GROUPING SETS((fact_1_id, fact_2_id), (fact_1_id, fact_3_id))
ORDER BY fact_1_id, fact_2_id, fact_3_id;

Composite Columns

ROLLUP and CUBE consider each column independently when deciding which subtotals must be calculated. For ROLLUP this means stepping back through the list to determine the groupings.

ROLLUP (a, b, c)
(a, b, c)
(a, b)
(a)
()

CUBE creates a grouping for every possible combination of columns.

CUBE (a, b, c)
(a, b, c)
(a, b)
(a, c)
(a)
(b, c)
(b)
(c)
()

Composite columns allow columns to be grouped together with braces so they are treated as a single unit when determining the necessary groupings. In the following ROLLUP columns "a" and "b" have been turned into a composite column by the additional braces. As a result the group of "a" is not longer calculated as the column "a" is only present as part of the composite column in the statement.

ROLLUP ((a, b), c)
(a, b, c)
(a, b)
()

Not considered:
(a)

In a similar way, the possible combinations of the following CUBE are reduced because references to "a" or "b" individually are not considered as they are treated as a single column when the groupings are determined.

CUBE ((a, b), c)
(a, b, c)
(a, b)
(c)
()

Not considered:
(a, c)
(a)
(b, c)
(b)

Concatenated Groupings

Concatenated groupings are defined by putting together multiple GROUPING SETS, CUBEs or ROLLUPs separated by commas. The resulting groupings are the cross-product of all the groups produced by the individual grouping sets. It might be a little easier to understand what this means by looking at an example. The following GROUPING SET results in 2 groups of subtotals, one for the fact_1_id column and one for the fact_id_2 column.

SELECT fact_1_id,
       fact_2_id,
       SUM(sales_value) AS sales_value,
       GROUPING_ID(fact_1_id, fact_2_id) AS grouping_id
FROM   dimension_tab
GROUP BY GROUPING SETS(fact_1_id, fact_2_id)
ORDER BY fact_1_id, fact_2_id;

 Fuente:  http://www.oracle-base.com/articles/misc/rollup-cube-grouping-functions-and-grouping-sets.php