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
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：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')
)
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。
