主要涉及:JOIN 、JOIN 更新、GROUP BY HAVING 數據查重/去重
1 INNER JOIN、LEFT JOIN、RIGHT JOIN、FULL JOIN(MySQL 不支持)、CROSS JOIN
這是在網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 的數據選取范圍
[][1]
[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_a SET col_a = 'hi' ;
3 查詢重復數據、刪除重復數據
利用 GROUP BY 和 HAVING 查詢重復數據
SELECT col_a, COUNT(*) FROM t1 GROUP BY col_a HAVING COUNT(*) > 1 ;
刪除重復數據,對於相同數據保留 ID 最大的
DELETE a FROM 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_a WHERE a.id < b.id ;
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