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：

select * from 插入表 left join 主表 on t1.lvid=t2.lv_id select * from mytable,title where 表名1.name=表名2.writer ;

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

$sql = "create temporary table tmp_table_name select `id`, `name`, `date`, '完成' as `type` from table_a where 条件语句……";

第二步：从table_b中取得相关记录插入临时表tmp_table_name中

insert into tmp_table_name select `id`, `name`, `date2` as `date`, '未完成' as `type` from table_b where 条件语句……

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')
 )

更多关于mysql相关内容感兴趣的读者可查看本站专题：《mysql查询技巧大全》、《mysql常用函数大汇总》、《mysql日志操作技巧大全》、《mysql事务操作技巧汇总》、《mysql存储过程技巧大全》及《mysql数据库锁相关技巧汇总》

希望本文所述对大家mysql数据库计有所帮助。