SQL : PostgreSQL
Aggregate Functions Like most other relational database products, PostgreSQL supports aggregate functions . An aggregate function computes a single result from multiple input rows. For example, there are aggregates to compute the count , sum , avg (average), max (maximum) and min (minimum) over a set of rows. As an example, we can find the highest low-temperature reading anywhere with: SELECT max(temp_lo) FROM weather; max ----- 46 (1 row) If we wanted to know what city (or cities) that reading occurred in, we might try: SELECT city FROM weather WHERE temp_lo = max(temp_lo); WRONG but this will not work since the aggregate max cannot be used in the WHERE clause. (This restriction exists because the WHERE clause determines which rows will be included in the aggregate calculation; so obviously it has to be evaluated before aggregate functions are computed.) However, as is o...
