PostgreSQL - HAVING 子句

HAVING 子句允许我们挑选出函数结果满足某些条件的特定行。

WHERE 子句对选定的列施加条件，而 HAVING 子句对 GROUP BY 子句创建的组施加条件。

语法

以下是 HAVING 子句在 SELECT 查询中的位置：

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

HAVING 子句必须跟在 GROUP BY 子句后面，并且如果使用 ORDER BY 子句，则也必须位于其前面。以下是包含 HAVING 子句的 SELECT 语句的语法：

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

示例

考虑表 COMPANY，其记录如下：

# 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)

以下是一个示例，它将显示名称计数小于 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 表中创建三个新记录：

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，我们的表具有以下包含重复名称的记录：

  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)

以下是一个示例，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)