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Approximates the number of distinct values for each given group.
APPROX_COUNT_DISTINCT([MAX_VALUES,] GROUP, VALUE)
Default: no limit
(Optional) The maximum number of groups. Groups beyond this amount are discarded.
In some cases, discarding groups via this option may cause some data to be discarded from groups that would otherwise have been kept.
The field by which to group the results.
The field to count distinct values relative to each group.
Returns an array of key-value pairs where each
GROUPis matched to the approximate number of distinct values
VALUEseen for that group.
If all input values are null, zero is returned.