主要涉及：JOIN 、JOIN 更新、GROUP BY HAVING 數(shù)據(jù)查重/去重

1 INNER JOIN、LEFT JOIN、RIGHT JOIN、FULL JOIN(MySQL 不支持)、CROSS JOIN

這是在網(wǎng)上找到的非常好的一篇博文，圖解 join 語句：

CODING HORROR-A Visual Explanation of SQL Joins

下圖可以很清楚的明白，join 的數(shù)據(jù)選取范圍

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[1]: http://7xs09x.com1.z0.glb.clouddn.com/160725-imooc-mysql-development-skills-notes-001.png

2 更新使用過濾條件中包括本身的表

更新 t1 t2 表中 col_a 重復的字段

UPDATE t1 SET col_a = 'hi' WHERE t1.col_a IN ( SELECT b.col_a FROM t1 a INNER JOIN t2 b on a.col_a = b.col_a);ERROR:1093 

可轉(zhuǎn)換為：

UPDATE t1 aa JOIN( SELECT b.col_a FROM t1 a INNER JOIN t2 b on a.col_a = b.col_a)bb on aa.col_a= bb.col_aSET col_a = 'hi' ;

3 查詢重復數(shù)據(jù)、刪除重復數(shù)據(jù)

利用 GROUP BY 和 HAVING 查詢重復數(shù)據(jù)

SELECT col_a, COUNT(*)FROM t1GROUP BY col_a HAVING COUNT(*) > 1;

刪除重復數(shù)據(jù)，對于相同數(shù)據(jù)保留 ID 最大的

DELETE aFROM t1 a JOIN ( SELECT col_a,COUNT(*),MAX(id) AS id FROM t1 GROUP BY col_a HAVING COUNT(*) > 1)b ON a.col_a = b.col_aWHERE a.id < b.id;

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