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dml ddl結(jié)構(gòu)語句查看全部
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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語句類型查看全部
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SQL語句類型查看全部
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分類聚合方式查詢每一個用戶某一個字段數(shù)據(jù)最大的兩條數(shù)據(jù)： 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優(yōu)化聚合子查詢： user1表 ---------------------------------- - id ---- user_name --- over ----- ---------------------------------- user_kills表 --------------------------------------- - id -- user_id -- kills -- timestr---- --------------------------------------- 問題：如何查詢出四人組中打怪最多的日期？ 一般思路：聚合子查詢 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優(yōu)化聚合子查詢 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優(yōu)化子查詢技巧： 一般子查詢寫法：(數(shù)據(jù)小時，沒有多大影響，如果數(shù)據(jù)量大時，則要消耗大量的查詢) select a.user_name , a.voer , (select over from user2 where a.user_name = b,user_name) as over2 from user1 a; 如果這兩張表的記錄相當(dāng)多 那么這個子查詢相當(dāng)于對A標(biāo)的每一條記錄都要進行一次子查詢。 join優(yōu)化（左連接）后的寫法： 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 聯(lián)合更新技巧： 如題：將user1表中user_name與user2表中user_name相同的user1表的over替換成user2表中的over值？ 一般正常思路： update user1 set over='齊天大圣' where user1.user_name in( select b.user_name from user1 a left join user2 b on a.user_name = b.user_name); 這樣去執(zhí)行思路上是對的，但是MySQL不支持這種機制 解決方法： 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='齊天大圣'查看全部
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分類聚合方式查詢每一個用戶某一個字段數(shù)據(jù)最大的兩條數(shù)據(jù)： 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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實現(xiàn)分組查看全部
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SQL開發(fā)技巧 row_number查看全部
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MySQL 實現(xiàn) Full?。剩铮椋?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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