PostgreSQL HAVING 子句-PostgreSQL入门教程

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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：
```
maxiaokedb# 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：
```
maxiaokedb-# SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) > 1;
```
得到结果如下：
```
 name
-------
 Paul
 James
(2 rows)
```

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