PostgreSQL HAVING 子句

HAVING 子句可以让我们筛选分组后的各组数据。

WHERE 子句在所选列上设置条件,而 HAVING 子句则在由 GROUP BY 子句创建的分组上设置条件。

语法

下面是 HAVING 子句在 SELECT 查询中的位置:

SELECT FROM WHERE GROUP BY HAVING ORDER BY

HAVING 子句必须放置于 GROUP BY 子句后面,ORDER BY 子句前面,下面是 HAVING 子句在 SELECT 语句中基础语法:

SELECT column1, column2 FROM table1, table2 WHERE [ conditions ] GROUP BY column1, column2 HAVING [ conditions ] ORDER BY column1, column2

实例

创建 COMPANY 表(下载 COMPANY SQL 文件),数据内容如下:

runoobdb# 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)

下面实例将找出根据 NAME 字段值进行分组,并且 name(名称) 字段的计数少于 2 数据:

SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) < 2;

得到以下结果:

 name ------- Teddy Paul Mark David Allen Kim James (7 rows)

我们往表里添加几条数据:

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);

此时,COMPANY 表的记录如下:

 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)

下面实例将找出根据 name 字段值进行分组,并且名称的计数大于 1 数据:

runoobdb-# SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) > 1;

得到结果如下:

 name ------- Paul James (2 rows)