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Note that the third column is a bitvector of columns being selected.
For the first row, none of the columns are being selected.
For the second row, only the first column is being selected, which explains the count of 2.
For the third row, both the columns are being selected (and the second column happens to be null), which explains the count of 1.

Cubes and Rollups

The general syntax is CUBE ( ). It is used with the GROUP BY only. CUBE creates a subtotal of all possible combinations of the set of column in its argument. Once we compute a CUBE on a set of dimension, we can get answer to all possible aggregation questions on those dimensions.

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