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Inner join查看全部
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join 方式查看全部
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sql语句类型查看全部
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SQL语句类型查看全部
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分类聚合方式查询每一个用户某一个字段数据最大的两条数据: select d.user_name ,c.ctimestr,kills from (select user_id ,timestr ,kills ,( select count(*) from user_kills b where b.user_id = a.user_id and a.kills <= b.kills) as cnt from user_kills a group by user_id,timestr,kills) c join user1 d on c.user_id = d.id where cnt <= 2查看全部
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MySQL 使用join优化聚合子查询: user1表 ---------------------------------- - id ---- user_name --- over ----- ---------------------------------- user_kills表 --------------------------------------- - id -- user_id -- kills -- timestr---- --------------------------------------- 问题:如何查询出四人组中打怪最多的日期? 一般思路:聚合子查询 select a.user_name , b.timestr , b.kills from user1 a join user_kills b on a.id = b.user_id where b.kills = (select MAX(c.kills) from user_kills c where c.user_id = b.user_id) 使用join + having优化聚合子查询 select a.user_name , b.timestr , b.kills from user1 a join user_kills b on a.id = b.user_id join user_kills c on c.user_id = b.user_id group by a.user_name , b.timestr , b.kills having b.kills = MAX(c.kills)查看全部
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join优化子查询技巧: 一般子查询写法:(数据小时,没有多大影响,如果数据量大时,则要消耗大量的查询) select a.user_name , a.voer , (select over from user2 where a.user_name = b,user_name) as over2 from user1 a; 如果这两张表的记录相当多 那么这个子查询相当于对A标的每一条记录都要进行一次子查询。 join优化(左连接)后的写法: select a.user_name , a.over , b.over from user1 a left join user2 b on a.user_name = b.user_name查看全部
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MySQL 联合更新技巧: 如题:将user1表中user_name与user2表中user_name相同的user1表的over替换成user2表中的over值? 一般正常思路: update user1 set over='齐天大圣' where user1.user_name in( select b.user_name from user1 a left join user2 b on a.user_name = b.user_name); 这样去执行思路上是对的,但是MySQL不支持这种机制 解决方法: update user1 a join ( select b.user_name from user1 a join user2 b on a.user_name = b.user_name ) b on a.user_name = b.user_name set a.over='齐天大圣'查看全部
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分类聚合方式查询每一个用户某一个字段数据最大的两条数据: select d.user_name ,c.ctimestr,kills from (select user_id ,timestr ,kills ,( select count(*) from user_kills b where b.user_id = a.user_id and a.kills <= b.kills) as cnt from user_kills a group by user_id,timestr,kills) c join user1 d on c.user_id = d.id where cnt <= 2查看全部
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实现分组查看全部
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SQL开发技巧 row_number查看全部
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MySQL 实现 Full Join查看全部
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Left Join查看全部
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inner join查看全部
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SQL语言分类查看全部
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