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)