SQL 中 SELECT 语句的语法

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在T-SQL中，SELECT语句的基本语法规则如下：

SELECT [ALL|DISTINCT] [TOP (<expression>) [PERCENT] [WITH TIES]] <column list> 
[FROM ] [WHERE <restrictive condition>] 
[GROUP BY <column name or expression using a column in the select list>] 
[HAVING <restrictive condition based on the group by results>] 
[ORDER BY <column list>] 
[[FOR XML {RAW|AUTO|EXPLICIT|PATH [(<element>)]}[, XMLDATA][, ELEMENTS] 
[, BINARY base 64]] 
[OPTION (<query hint>, [, ...n])]

其中HAVING子句是给分组设置条件的，与WHERE子句的功能一样，只是用在不同的地方。HAVING子句仅用于带有GROUP BY子句的查询语句中。WHERE子句应用于每一行（在变成一组的某一部分之前），而HAVING子句应用于分组的聚合值。如果要将查询条件放到分组之后，可以使用HAVING子句。

以下是两个对比例子：

SELECT ManagerID AS Manager, COUNT(*) AS Reports
FROM HumanResources.Employee
WHERE EmployeeID != 5
GROUP BY ManagerID;

返回的结果为：

Manager Reports

--------------------------------

NULL 1

1 3

4 2

5 4

（4 行受影响）

SELECT ManagerID AS Manager, COUNT(*) AS Reports
FROM HumanResources.Employee
WHERE EmployeeID != 5
GROUP BY ManagerID
HAVING COUNT(*) > 3;

结果为：

Manager Reports

-----------------------------

5 4

（1行受影响）