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/*INNER JOIN*/ SELECT a.`user_name`,b.`id` FROM friends a /*用a代替friends*/ INNER JOIN qujing b on a.`user_name`=b.`user_name` /*非join*/ SELECT a.id,b.user_name FROM friends a,qujing b WHERE a.`user_name`=b.user_name查看全部
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笛卡尔积---cross join 交叉连接查看全部
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join的分类查看全部
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结构化查询语言 mysql查看全部
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mysql 结构化查看全部
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mysql分组查询： select d.user_name,c.timestr,kills FROM (select user_id,timrstr,kills,(SELECT COUNTA(*) FROM user_kils 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 dON c.user_id=d.id WHERE cnt <= 2查看全部
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join优化聚合子查询： -- 使用join优化聚合子查询 -- 如何查询tangtang表中打怪最多的日期 -- select a.user_name ,b.timestr,b.kills from tangtang a join killscount b on a.id = b.user_id -- where b.kills=(select max(c.kills) from killscount c where c.user_id = b.user_id) -- select * from killscount b join tangtang a on b.user_id = a.id order by kills DESC -- select a.user_name,b.timestr,max(b.kills) from tangtang a join killscount b on a.id = b.user_id group by a.user_name,b.timestr; -- 优化后语句 避免子查询 -- select a.user_name,b.timestr,b.kills from tangtang a -- join killscount b on a.id = b.user_id -- join killscount 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： 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.over , (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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使用join更新表： 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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cross join： SELECT * from user1 a cross join user2 b;查看全部
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子查询是比较低效地，因为每一条记录都要进行匹配，如果记录长度比较大的话，那么我们的查询就有可能非常的耗时查看全部
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常见的SQL语句类型查看全部
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结构化查询语句查看全部
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分组 查询 查出每组前两名查看全部
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join五种连接查看全部
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