PostgreSQL HAVING 子句

PostgreSQL HAVING 子句

HAVING 子句可以讓我們篩選分組後的各組資料。

WHERE 子句在所選列上設定條件，而 HAVING 子句則在由 GROUP BY 子句建立的分組上設定條件。

語法

下面是 HAVING 子句在 SELECT 查詢中的位置：

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

HAVING 子句必須放置於 GROUP BY 子句後面，ORDER BY 子句前面，下面是 HAVING 子句在 SELECT 語句中基礎語法：

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

例項

建立 COMPANY 表（下載 COMPANY SQL 檔案 ），資料內容如下：

itread01db# 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)

下面例項將找出根據 NAME 欄位值進行分組，並且 name(名稱) 欄位的計數少於 2 資料：

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

得到以下結果：

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

我們往表裡新增幾條資料：

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);

此時，COMPANY 表的記錄如下：

 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)

下面例項將找出根據 name 欄位值進行分組，並且名稱的計數大於 1 資料：

itread01db-# SELECT NAME FROM COMPANY GROUP BY name HAVING count(name) > 1;

得到結果如下：

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