The aggregate functions can be used in a SELECT list, as in the previous examples, or in the HAVING clause of a select statement that includes a GROUP BY clause.
You cannot use aggregate functions in a WHERE clause or in a JOIN condition. However, a SELECT statement with aggregate functions in its SELECT list often includes a WHERE clause that restricts the rows to which the aggregate is applied.
If a SELECT statement includes a WHERE clause, but not a GROUP BY clause, an aggregate function produces a single value for the subset of rows that the WHERE clause specifies.
Whenever an aggregate function is used in a SELECT statement that does not include a GROUP BY clause, it produces a single value. This is true whether it is operating on all the rows in a table or on a subset of rows defined by a where clause.
You can use more than one aggregate function in the same SELECT list, and produce more than one scalar aggregate in a single SELECT statement.
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