mysql、oracle、pg、sql查询、删除重复数据保留一条

1、单个字段查询、去重

查询单字段重复数据

select * from 表名称 
where 字段 in (select 字段 from 表名称 group by 字段 having count(1) > 1)

单个字段去重，保留一条

注意count(字段)会忽略空值，如果空值也要去重使用count(1) 或者 count(*)。

效率count(主键) > count(1) > count(字段) > coung(*)

min可以换max，oracle中也可以用rowid

delete from 表名称 
where 字段 in (select 字段 from 表名称 group by 字段 having count(1) > 1)
and id not in (select min(id) from 表名称 group by 字段 having count(1) > 1)

2、多字段查询、去重

查询多字段重复数据查询

select * from 表名称 
where (字段1,字段2) in (select 字段1,字段2 from 表名称 group by 字段1,字段2 having count(1) > 1)

多个字段去重，保留一条

注意：此方法不能去除 字段1,字段2 同时为null的情况，同时为null需要另外写sql

delete from 表名称 
where (字段1,字段2) in (select 字段1,字段2 from 表名称 group by 字段1,字段2 having count(1) > 1)
and id not in (select min(id) from 表名称 group by 字段1,字段2 having count(1) > 1)

多字段同时为null，保留一条

delete from 表名称 
where 字段1 is null and 字段2 is null 
and id not in (select min(id) from test where 字段1 is null and 字段2 is null )