The general form of the SELECT statement is further enhanced by the addition of the HAVING clause and becomes:
SELECT column|expression|group_function(column|expression [alias]), ...} FROM table
[WHERE condition(s)]
[GROUP BY {col(s)|expr}]
[HAVING group_condition(s)]
[ORDER BY {col(s)|expr|numeric_pos} [ASC|DESC] [NULLS FIRST|LAST]];
An important difference between the HAVING clause and the other SELECT statement clauses is that it may only be specified if a GROUP BY clause is present. This dependency is sensible since group-level rows must exist before they can be restricted. The HAVING clause can occur before the GROUP BY clause in the SELECT statement. However, it is more common to place the HAVING clause after the GROUP BY clause. All grouping is performed and group functions are executed prior to evaluating the HAVING clause.