MySQL 数据库删除表中的重复行

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/weixin_37879830/article/details/88249622

1.查找重复记录

1）查询两表的差异行：应用场景：查询增量数据

SELECT id,title
FROM (
    SELECT id, title FROM t1
    UNION ALL
    SELECT id,title FROM t2
) tbl
GROUP BY id, title
HAVING count(*) = 1
ORDER BY id;

注意：having 的作用方对group by的每个小组应用条件

2）找本表中的重复值：找一列中重复记录

SELECT 
    col, 
    COUNT(col)
FROM
    table_name
GROUP BY col
HAVING COUNT(col) > 1;

3）本表重复值：找几个组合列中重复记录

SELECT 
    col1, COUNT(col1),
    col2, COUNT(col2),
    ...

FROM
    table_name
GROUP BY 
    col1, 
    col2, ...
HAVING 
       (COUNT(col1) > 1) AND 
       (COUNT(col2) > 1) AND 
       ...

2.删除重复记录

DELETE T1 FROM CONTACTS T1 INNER JOIN CONTACTS T2 WHERE T1.ID<T2.ID AND T1.E_MAIL=T2.E_MAIL;