mysql中如何实现多表联合查询运作

发布时间：2022-02-13 06:37:45 所属栏目：MySql教程来源：互联网

导读：这篇文章将为大家详细讲解有关mysql中如何实现多表联合查询操作，文章内容质量较高，因此小编分享给大家做个参考，希望大家阅读完这篇文章后对相关知识有一定的了解。复制代码代码如下: SELECT * FROM 插入表 LEFT JOIN 主表 ON t1.lvid=t2.lv_id select *

这篇文章将为大家详细讲解有关mysql中如何实现多表联合查询操作，文章内容质量较高，因此小编分享给大家做个参考，希望大家阅读完这篇文章后对相关知识有一定的了解。

复制代码代码如下:

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

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 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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