Using GROUPING SETS is useful when you want to concatenate several different data partitions into a single result set. However, if you have many groupings to specify, and want subtotals included, you may want to use the ROLLUP and CUBE extensions.
The ROLLUP and CUBE clauses can be considered shortcuts for pre-defined GROUPING SETS specifications.
ROLLUP is equivalent to specifying a series of grouping set specifications starting with the empty grouping set '()' and successively followed by grouping sets where one additional expression is concatenated to the previous one. For example, if you have three grouping expressions, a, b, and c, and you specify ROLLUP, it is as though you specified a GROUPING SETS clause with the sets: (), (a), (a, b), and (a, b, c ). This construction is sometimes referred to as hierarchical groupings.
CUBE offers even more groupings. Specifying CUBE is equivalent to specifying all possible GROUPING SETS. For example, if you have the same three grouping expressions, a, b, and c, and you specify CUBE, it is as though you specified a GROUPING SETS clause with the sets: (), (a), (a, b), (a, c), (b), (b, c), (c), and (a, b, c ).
When specifying ROLLUP or CUBE, use the GROUPING function to distinguish placeholder NULLs in your results, caused by the subtotal rows that are implicit in a result set formed by ROLLUP or CUBE. See Detecting placeholder NULLs using the GROUPING function.
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