SQL Server GROUP BY 关键字

SQL Server (Transact-SQL) GROUP BY 关键字用于借助聚合函数 (Count，Max，MIN, SUM, AVG 等）进行查询。 GROUP BY 关键字位于 SELECT 语句中的 WHERE 子句之后，位于 ORDER BY 关键字之前。

语法

在 SQL Server (Transact-SQL) 中使用 GROUP BY 关键字的语法如下:

SELECT column1, column2
FROM table_name
WHERE condition(s)
GROUP BY column1, column2
ORDER BY column1, column2; 

示例:

考虑名为 Employee 的数据库表，包含以下记录:

GROUP BY 单列:要查找 Employee 表中按年龄（降序）排序的不同年龄段的员工人数，查询为:

SELECT Age, Count(Name) AS Number_of_Employee
FROM Employee
GROUP BY Age
ORDER BY Age DESC; 

这将产生如下结果:

GROUP BY 多列: 查找Employee 按城市和年龄排序的表，查询为:

SELECT City, Age, AVG(Salary) AS AverageSalary
FROM Employee
GROUP BY City, Age
ORDER BY City, Age; 

这将产生如下所示的结果: