在哪里与HAVING

Question

为什么你需要把你自己创建的(例如列select 1 as "number"后)HAVING,而不是WHERE在MySQL？

是否有任何缺点而不是做WHERE 1(编写整个定义而不是列名)？

Answer 1

为什么你需要在HAVING之后放置你自己创建的列(例如"select 1 as number")而不是MySQL中的WHERE？

WHERE被施加之前GROUP BY,HAVING被后应用(并且可以在聚集体筛选).

在一般情况下,你可以在没有这些条款的引用别名,但MySQL可以参考SELECT水平别名GROUP BY,ORDER BY和HAVING.

是否有任何缺点而不是做"WHERE 1"(写整个定义而不是列名)

如果计算的表达式不包含任何聚合,则将其放入WHERE子句中可能会更有效.

Answer 2

所有的答案都没有达到关键点.

假设我们有一张桌子:

CREATE TABLE `table` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `value` int(10) unsigned NOT NULL,
 PRIMARY KEY (`id`),
 KEY `value` (`value`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

并且有10行,id和值从1到10:

INSERT INTO `table`(`id`, `value`) VALUES (1, 1),(2, 2),(3, 3),(4, 4),(5, 5),(6, 6),(7, 7),(8, 8),(9, 9),(10, 10);

尝试以下2个查询:

SELECT `value` v FROM `table` WHERE `value`>5; -- Get 5 rows
SELECT `value` v FROM `table` HAVING `value`>5; -- Get 5 rows

您将获得完全相同的结果,您可以看到HAVING子句可以在没有GROUP BY子句的情况下工作.

这是区别:

SELECT `value` v FROM `table` WHERE `v`>5;

错误#1054 - 'where子句'中的未知列'v'

SELECT `value` v FROM `table` HAVING `v`>5; -- Get 5 rows

WHERE子句要求条件是表中的列,但HAVING子句可以使用列或别名.

这是因为WHERE子句在select之前过滤数据,但是HAVING子句在select之后过滤数据.

因此,如果表中有许多行,则将WHERE子句中的条件更有效.

尝试使用EXPLAIN查看关键区别:

EXPLAIN SELECT `value` v FROM `table` WHERE `value`>5;
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+
|  1 | SIMPLE      | table | range | value         | value | 4       | NULL |    5 | Using where; Using index |
+----+-------------+-------+-------+---------------+-------+---------+------+------+--------------------------+

EXPLAIN SELECT `value` v FROM `table` having `value`>5;
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
|  1 | SIMPLE      | table | index | NULL          | value | 4       | NULL |   10 | Using index |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+

您可以看到WHERE或HAVING使用索引,但行不同.

Answer 3

主要区别在于WHERE不能用于分组项目(例如SUM(number))而HAVING可以.

原因是WHERE完成之前的分组和HAVING完成后的分组完成.

Answer 4

HAVING用于过滤您的聚合GROUP BY.

例如,要检查重复的名称:

SELECT Name FROM Usernames
GROUP BY Name
HAVING COUNT(*) > 1

Answer 5

这两个将与第一个相同,因为两者都用于说明过滤数据的条件.虽然我们可以在任何情况下使用'have'来代替'where',但有些情况下我们不能使用'where'而不是'having'.这是因为在选择查询中,"where"在'select'之前过滤数据,而在'select'之后'过滤'数据.因此,当我们使用实际上不在数据库中的别名时,"where"无法识别它们,但"有"可以.

例如:让学生表包含student_id,姓名,生日,地址.假设生日是日期类型.

SELECT * FROM Student WHERE YEAR(birthday)>1993; /*this will work as birthday is in database.if we use having in place of where too this will work*/

SELECT student_id,(YEAR(CurDate())-YEAR(birthday)) AS Age FROM Student HAVING Age>20; 
/*this will not work if we use ‘where’ here, ‘where’ don’t know about age as age is defined in select part.*/