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` >= ‘2022-11-22’)

  )

AND (`date` <= ‘2022-02-20’)

)

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` >= ‘2022-11-22’)

   )

  AND (`date` <= ‘2022-02-20’)

)

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` >= ‘2022-11-22’)

   )

  AND (`date` <= ‘2022-02-20’)

)

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` >= ‘2022-11-22’)

   )

  AND (`date` <= ‘2022-02-20’)

)

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` >= ‘2022-11-22’)

   )

  AND (`date` <= ‘2022-02-20’)

)

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希望本文所述对大家MySQL数据库计有所帮助。