mysql多表联合查询操作实例分析

2020-01-18 20:50:52

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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…… UNIONSELECT `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

复制代码代码如下:

$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

复制代码代码如下:

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 nameunionselect * from test1 where name like 'B%' order by name

修改为：

代码示例:

select * from test1 where name like 'A%'unionselect * 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 10unionselect * 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') )