Aggregate functions operate on sets of values. 聚合函数对值集进行操作。They are often used with a GROUP BY clause to group values into subsets. 它们通常与GROUP BY子句一起用于将值分组到子集中。This section describes most aggregate functions. 本节介绍大多数聚合函数。For information about aggregate functions that operate on geometry values, see Section 12.17.12, “Spatial Aggregate Functions”.有关操作几何值的聚合函数的信息，请参阅第12.17.12节“空间聚合函数”。