COUNT is an aggregate function that counts the number of rows
accessed in an expression (see
The 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
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