Syntax COUNT ( [ DISTINCT | ALL ] Expression ) DISTINCTqualifier
in COUNT aggregate functionThe DISTINCT qualifier
eliminates duplicates. The ALL qualifier retains duplicates. ALL is assumed
if neither ALL nor DISTINCT is specified. For example, if a column contains
the values 1, 1, 1, 1, and 2, COUNT(col) returns a greater value than COUNT(DISTINCT
col).
Only one DISTINCT aggregate expression per SelectExpression is
allowed. For example, the following query is not allowed:
-- query not allowed
SELECT COUNT (DISTINCT flying_time), SUM (DISTINCT miles)
FROM Flights An Expression can contain multiple
column references or expressions, but it cannot contain another aggregate
or subquery. If an Expression evaluates to NULL, the aggregate is not
processed for that value.
The resulting data type of COUNT is BIGINT.