本文实例讲述了mysql多表联合查询操作。分享给大家供大家参考，具体如下：
MySQL多表联合查询是MySQL数据库的一种查询方式，下面就为您介绍MySQL多表联合查询的语法，供您参考学习之用。
MySQL多表联合查询语法：
mysql版本大于4.0，使用UNION进行查询，示例如下：
SELECT `id`, `name`, `date`, '' AS `type` FROM table_A WHERE 条件语句…… UNION SELECT `id`, `name`, `date`, '未完成' AS `type` FROM table_B WHERE 条件语句…… ORDER BY `id` LIMIT num;
mysql版本小于4.0，需要建立临时表，分为三步，示例如下：
第一步：建立临时表tmp_table_name并插入table_A中的相关记录
第二步：从table_B中取得相关记录插入临时表tmp_table_name中
第三步：从临时表tmp_table_name中取出记录
SELECT * FROM tmp_table_name ORDER BY id DESC
union和order by、limit区别分析
代码示例:
CREATE TABLE `test1` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `name` varchar(20) NOT NULL, `desc` varchar(100) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8
1. 以下查询会报错误：[Err] 1221 - Incorrect usage of UNION and ORDER BY
代码示例:
select * from test1 where name like 'A%' order by name union select * from test1 where name like 'B%' order by name
修改为：
代码示例:
select * from test1 where name like 'A%' union select * from test1 where name like 'B%' order by name
说明，在union中，不用括号的情况下，只能用一个order by(思考：union两边的order by的列名不一样时，会出现什么样的结果？),这会对union后的结果集进行排序。
修改为：
代码示例:
(select * from test1 where name like 'A%' order by name) union (select * from test1 where name like 'B%' order by name)
也是可以的，这两个order by在union前进行。
2. 同样
代码示例:
select * from test1 where name like 'A%' limit 10 union select * from test1 where name like 'B%' limit 20
相当于：
代码示例:
(select * from test1 where name like 'A%' limit 10) union (select * from test1 where name like 'B%') limit 20
即后一个limit作用于的是union后的结果集，而不是union后的select。
也可以用括号括起来，以得到预期的结果：
3. UNION和UNION ALL区别
union会过滤掉union两边的select结果集中的重复的行，而union all不会过滤掉重复的行。
代码示例:
(select * from test1 where name like 'A%' limit 10) union (select * from test1 where name like 'B%' limit 20)
下面试一个年龄段分析的复杂sql语句
(
SELECT
'5~19' AS `age`,
SUM(`impression`) AS impression,
SUM(`click`) AS click,
sum(`cost`) AS cost
FROM
`adgroup_age_report`
WHERE
(
(
(`age` <= 19)
AND (`adgroup_id` = '61')
)
AND (`date` >= '2015-11-22')
)
AND (`date` <= '2017-02-20')
)
UNION
(
SELECT
'20~29' AS `age`,
SUM(`impression`) AS impression,
SUM(`click`) AS click,
sum(`cost`) AS cost
FROM
`adgroup_age_report`
WHERE
(
(
((`age` <= 29) AND(`age` >= 20))
AND (`adgroup_id` = '61')
)
AND (`date` >= '2015-11-22')
)
AND (`date` <= '2017-02-20')
)
UNION
(
SELECT
'30~39' AS `age`,
SUM(`impression`) AS impression,
SUM(`click`) AS click,
sum(`cost`) AS cost
FROM
`adgroup_age_report`
WHERE
(
(
((`age` <= 39) AND(`age` >= 30))
AND (`adgroup_id` = '61')
)
AND (`date` >= '2015-11-22')
)
AND (`date` <= '2017-02-20')
)
UNION
(
SELECT
'40~49' AS `age`,
SUM(`impression`) AS impression,
SUM(`click`) AS click,
sum(`cost`) AS cost
FROM
`adgroup_age_report`
WHERE
(
(
((`age` <= 49) AND(`age` >= 40))
AND (`adgroup_id` = '61')
)
AND (`date` >= '2015-11-22')
)
AND (`date` <= '2017-02-20')
)
UNION
(
SELECT
'50~59' AS `age`,
SUM(`impression`) AS impression,
SUM(`click`) AS click,
sum(`cost`) AS cost
FROM
`adgroup_age_report`
WHERE
(
(
((`age` <= 59) AND(`age` >= 50))
AND (`adgroup_id` = '61')
)
AND (`date` >= '2015-11-22')
)
AND (`date` <= '2017-02-20')
)
希望本文所述对大家MySQL数据库计有所帮助。
