SQL HAVING 子句

SQL HAVING 子句

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​HAVING​ 子句使你能够指定过滤条件，从而控制查询结果中哪些组可以出现在最终结果里面。

​WHERE ​子句对被选择的列施加条件，而 ​HAVING ​子句则对 ​GROUP BY​ 子句所产生的组施加条件。

语法  

下面可以看到 ​HAVING​ 子句在 ​SEL ECT ​查询中的位置：

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

在 ​SELECT​ 查询中，​HAVING​ 子句必须紧随 ​GROUP BY​ 子句，并出现在 ​ORDER BY​ 子句（如果有的话）之前。带有 ​HAVING​ 子句的 ​SELECT​ 语句的语法如下所示：

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

示例

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　考虑 ​CUSTOMERS​ 表，表中的记录如下所示：

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

　下面是一个有关 ​HAVING​ 子句使用的实例，该实例将会筛选出出现次数大于或等于 2 的所有记录。

SQL > SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM CUSTOMERS
GROUP BY age
HAVING COUNT(age) >= 2;

　其执行结果如下所示：

+----+----------+-----+---------+---------+
| ID | NAME     | AGE | ADDRESS | SALARY  |
+----+----------+-----+---------+---------+
|  2 | Khilan   |  25 | Delhi   | 1500.00 |
|  4 | Chaitali |  25 | Mumbai  | 6500.00 |
+----+----------+-----+---------+---------+

SELECT Websites.name, Websites.url, SUM(access_log.count) AS nums FROM (access_log 
INNER JOIN Websites ON access_log.site_id=Websites.id)
 GROUP BY Websites.name
 HAVING SUM(access_log.count) > 200;

SELECT Websites.name, SUM(access_log.count) AS nums FROM Websites 
INNER JOIN access_log 
ON Websites.id=access_log.site_id 
WHERE Websites.alexa < 200 
GROUP BY Websites.name 
HAVING SUM(access_log.count) > 200;