mysql中如何实现多表联合查询运作
发布时间：2022-02-12 18:39:39 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 *
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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') ) 关于mysql中如何实现多表联合查询操作就分享到这里了，希望以上内容可以对大家有一定的帮助，可以学到更多知识。如果觉得文章不错，可以把它分享出去让更多的人看到。 （编辑：晋中站长网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
