每当我们在sql(MIN,MAX,AVG等)中使用聚合函数时,我们必须始终GROUP BY所有非聚合列,例如：
>如果不是这样,那么GROUP BY给我们什么额外的功能？
SELECT storeid,storename,SUM(revenue),COUNT(*) FROM Sales GROUP BY storeid,storename@H_403_4@当我们在SELECT语句中使用函数或其他计算时,它变得更加突出,因为这也必须复制到GROUP BY子句.
SELECT (2 * (x + y)) / z + 1,MyFunction(x,y),SUM(z) FROM AnotherTable GROUP BY (2 * (x + y)) / z + 1,y)@H_403_4@如果我们改变了SELECT语句,我们必须记住对GROUP BY子句进行相同的更改. @H_403_4@GROUP BY子句是多余的吗？ @H_403_4@>如果确实是这样,那么为什么sql中有一个GROUP BY子句呢？
>如果不是这样,那么GROUP BY给我们什么额外的功能？
解决方法
@H_403_4@Whenever we use an aggregate function in sql (MIN,AVG etc),we must always GROUP BY all non-aggregated columns@H_403_4@一般来说这是不正确的.例如MysqL不需要这个,而且sql标准也没有这么说. @H_403_4@> Debunking GROUP BY myths
@H_403_4@It becomes even more intrusive when we use a function or other calculation in our SELECT statement,as this must also be copied to the GROUP BY clause.@H_403_4@一般也不正确. MysqL(也许其他数据库)也允许在GROUP BY子句中使用列别名：
SELECT (2 * (x + y)) / z + 1 AS a,y) AS b,SUM(z) FROM AnotherTable GROUP BY a,b
@H_403_4@If this is not the case,then what extra functionality does GROUP BY give us?@H_403_4@指定要分组的唯一方法是使用GROUP BY子句.您不一定会从SELECT中提到的列中推断出它.实际上你甚至不必选择GROUP BY中提到的所有列：
SELECT MAX(col2) FROM foo GROUP BY col1 HAVING COUNT(*) = 2
