mysql多表联合查询操作实例分析
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') )
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