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四種類型查看全部
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sql:結(jié)構(gòu)化查詢語(yǔ)言查看全部
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John從句得類型： （1）內(nèi)鏈接（ INNER ） （2）全外連接（ FULL OUTER ） （3）左外連接（ LEFT OUTER ） （4）右外連接（ RIGHT OUTER ） （5）交叉鏈接（ CROSS ）查看全部
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sql語(yǔ)句的使用的作用查看全部
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看到j(luò)oin從句左外連接查看全部
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分類聚合方式查詢每一個(gè)用戶某一個(gè)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(wèn)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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低效的子查詢 select a.user_name,a.over,(select over from user2 b where a.user_name=b.user_name) as over2 from user1 a; 使用join優(yōu)化后的子查詢： select a.user_name,a.over,b.over as over2 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 join user2 b on a.user_name = b.user_name); 這樣去執(zhí)行思路上是對(duì)的，但是MySQL不支持這種機(jī)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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發(fā)展分類查看全部
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分類聚合方式查詢每一個(gè)用戶某一個(gè)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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update user1 set over=‘齊天大圣’ where user1.user_name in ( select b.user_name from user1 a join user2 b on a.user_name = b.user_name); ERROR 1093錯(cuò)誤 不能更新from語(yǔ)句中出現(xiàn)的表字段 解決辦法: 通過(guò)使用join進(jìn)行聯(lián)合更新 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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交叉連接（笛卡爾積連接）：A*B，其實(shí)就是將兩個(gè)表進(jìn)行相乘,一般項(xiàng)目中很少用到笛卡爾積連接 select * from user1 a cross join user2 b; 如上語(yǔ)句：如果user1表中有3條記錄，user2中有4條記錄，則結(jié)果就有12條記錄查看全部
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MySQL 中 不會(huì)支持 full join 解決方法：采用左連接和右連接結(jié)合+ union all 方法來(lái)取得兩表的合集 select a.user_name , a.over , b.over <-這里是指所選擇顯示的項(xiàng) from user1 a left join user2 b on a.user_name = b.user_name <-這里是指篩選條件 union all <-連接反向查詢語(yǔ)句 select b.user_name , b.over, a.over <-這里是指所選擇顯示的項(xiàng) from user1 a right join user2 b on a.user_name = b.user_name <-這里是指篩選條件查看全部
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