Postgresql 简明教程
PostgreSQL - HAVING Clause
HAVING 子句允许我们选择特定行,其中函数结果满足某个条件。
WHERE 子句对所选列设置条件,而 HAVING 子句对 GROUP BY 子句创建的组设置条件。
Syntax
以下是在 SELECT 查询中 HAVING 子句的位置 −
SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY
HAVING 子句必须跟随查询中的 GROUP BY 子句,且如果使用,还必须位于 ORDER BY 子句之前。以下是 SELECT 语句的语法,其中包括 HAVING 子句 −
SELECT column1, column2
FROM table1, table2
WHERE [ conditions ]
GROUP BY column1, column2
HAVING [ conditions ]
ORDER BY column1, column2
Example
考虑 COMPANY 表具有以下记录:
# select * from COMPANY;
id | name | age | address | salary
----+-------+-----+-----------+--------
1 | Paul | 32 | California| 20000
2 | Allen | 25 | Texas | 15000
3 | Teddy | 23 | Norway | 20000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
6 | Kim | 22 | South-Hall| 45000
7 | James | 24 | Houston | 10000
(7 rows)
以下为一个示例,它将显示名称计数少于 2 的记录 −
testdb-# SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) < 2;
这将产生以下结果 -
name
-------
Teddy
Paul
Mark
David
Allen
Kim
James
(7 rows)
现在,让我们使用以下 INSERT 语句在 COMPANY 表中创建三条更多记录 −
INSERT INTO COMPANY VALUES (8, 'Paul', 24, 'Houston', 20000.00);
INSERT INTO COMPANY VALUES (9, 'James', 44, 'Norway', 5000.00);
INSERT INTO COMPANY VALUES (10, 'James', 45, 'Texas', 5000.00);
现在,我们的表有以下具有重复名称的记录 −
id | name | age | address | salary
----+-------+-----+--------------+--------
1 | Paul | 32 | California | 20000
2 | Allen | 25 | Texas | 15000
3 | Teddy | 23 | Norway | 20000
4 | Mark | 25 | Rich-Mond | 65000
5 | David | 27 | Texas | 85000
6 | Kim | 22 | South-Hall | 45000
7 | James | 24 | Houston | 10000
8 | Paul | 24 | Houston | 20000
9 | James | 44 | Norway | 5000
10 | James | 45 | Texas | 5000
(10 rows)
以下为一个示例,它将显示名称计数大于 1 的记录 −
testdb-# SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) > 1;
这将产生以下结果 -
name
-------
Paul
James
(2 rows)