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 子句在选定的列上放置条件，而 HAVING 子句在由 GROUP BY 子句创建的组上放置条件。

句法

以下是 SELECT 查询中 HAVING 子句的位置 –

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

HAVING 子句必须跟在查询中的 GROUP BY 子句之后，并且如果使用，还必须在 ORDER BY 子句之前。以下是 SELECT 语句的语法，包括 HAVING 子句 –

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

例子

考虑表COMPANY 的记录如下 –

# 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 表中再创建三个记录 –

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)