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：
代码如下: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
代码如下:$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
代码如下: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。

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