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数据库MySQL如何查找删除重复行
- MYSQL
- 2019-01-19
简介当你对b字段排序(分组),相同值的c被分到不同的组,因此不能用COUNT(DISTINCT c)来计算大小。COUNT()之类的内部函数只作用于同一个分组,对于不同分组的行就无能为力了。类似,如果排序的是c字字数
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当你对b字段排序(分组),相同值的c被分到不同的组,因此不能用COUNT(DISTINCT c)来计算大小。COUNT()之类的内部函数只作用于同一个分组,对于不同分组的行就无能为力了。类似,如果排序的是c字段,相同值的b也会分到不同的组,无论如何是不能达到我们的目的的。
几种正确的方法
也许最简单的方法是分别对某个字段查找重复行,然后用UNION拼在一起,像这样:
-
select b as value, count(*) as cnt, 'b' as what_col
-
from a_b_c group by b having count(*) > 1
-
union
-
select c as value, count(*) as cnt, 'c' as what_col
-
from a_b_c group by c having count(*) > 1;
-
+-------+-----+----------+
-
| value | cnt | what_col |
-
+-------+-----+----------+
-
| 1 | 3 | b |
-
| 2 | 3 | b |
-
| 3 | 3 | b |
-
| 1 | 3 | c |
-
| 2 | 3 | c |
-
| 3 | 3 | c |
-
+-------+-----+----------+
输出what_col字段为了提示重复的是哪个字段。另一个办法是使用嵌套查询:
-
select a, b, c from a_b_c
-
where b in (select b from a_b_c group by b having count(*) > 1)
-
or c in (select c from a_b_c group by c having count(*) > 1);
-
+----+------+------+
-
| a | b | c |
-
+----+------+------+
-
| 7 | 1 | 1 |
-
| 8 | 1 | 2 |
-
| 9 | 1 | 3 |
-
| 10 | 2 | 1 |
-
| 11 | 2 | 2 |
-
| 12 | 2 | 3 |
-
| 13 | 3 | 1 |
-
| 14 | 3 | 2 |
-
| 15 | 3 | 3 |
-
+----+------+------+
这种方法的效率要比使用UNION低许多,并且显示每一重复的行,而不是重复的字段值。还有一种方法,将自己跟group的嵌套查询结果联表查询。写法比较复杂,但对于复杂的数据或者对效率有较高要求的情况,是很有必要的。
-
select a, a_b_c.b, a_b_c.c
-
from a_b_c
-
left outer join (
-
select b from a_b_c group by b having count(*) > 1
-
) as b on a_b_c.b = b.b
-
left outer join (
-
select c from a_b_c group by c having count(*) > 1
-
) as c on a_b_c.c = c.c
-
where b.b is not null or c.c is not null
以上方法可行,我敢肯定还有其他的方法。如果UNION能用,我想会是最简单不过的了。
转载:
感谢您对莫愁个人博客网站平台的认可,非常欢迎各位朋友分享到个人站长或者朋友圈,但转载请说明文章出处“来源莫愁个人博客 https://www.mochoublog.com/study/68.html”。
- MYSQL
- 2019-01-19
当你对b字段排序(分组),相同值的c被分到不同的组,因此不能用COUNT(DISTINCT c)来计算大小。COUNT()之类的内部函数只作用于同一个分组,对于不同分组的行就无能为力了。类似,如果排序的是c字段,相同值的b也会分到不同的组,无论如何是不能达到我们的目的的。
几种正确的方法
也许最简单的方法是分别对某个字段查找重复行,然后用UNION拼在一起,像这样:
-
select b as value, count(*) as cnt, 'b' as what_col
-
from a_b_c group by b having count(*) > 1
-
union
-
select c as value, count(*) as cnt, 'c' as what_col
-
from a_b_c group by c having count(*) > 1;
-
+-------+-----+----------+
-
| value | cnt | what_col |
-
+-------+-----+----------+
-
| 1 | 3 | b |
-
| 2 | 3 | b |
-
| 3 | 3 | b |
-
| 1 | 3 | c |
-
| 2 | 3 | c |
-
| 3 | 3 | c |
-
+-------+-----+----------+
输出what_col字段为了提示重复的是哪个字段。另一个办法是使用嵌套查询:
-
select a, b, c from a_b_c
-
where b in (select b from a_b_c group by b having count(*) > 1)
-
or c in (select c from a_b_c group by c having count(*) > 1);
-
+----+------+------+
-
| a | b | c |
-
+----+------+------+
-
| 7 | 1 | 1 |
-
| 8 | 1 | 2 |
-
| 9 | 1 | 3 |
-
| 10 | 2 | 1 |
-
| 11 | 2 | 2 |
-
| 12 | 2 | 3 |
-
| 13 | 3 | 1 |
-
| 14 | 3 | 2 |
-
| 15 | 3 | 3 |
-
+----+------+------+
这种方法的效率要比使用UNION低许多,并且显示每一重复的行,而不是重复的字段值。还有一种方法,将自己跟group的嵌套查询结果联表查询。写法比较复杂,但对于复杂的数据或者对效率有较高要求的情况,是很有必要的。
-
select a, a_b_c.b, a_b_c.c
-
from a_b_c
-
left outer join (
-
select b from a_b_c group by b having count(*) > 1
-
) as b on a_b_c.b = b.b
-
left outer join (
-
select c from a_b_c group by c having count(*) > 1
-
) as c on a_b_c.c = c.c
-
where b.b is not null or c.c is not null
以上方法可行,我敢肯定还有其他的方法。如果UNION能用,我想会是最简单不过的了。
转载: 感谢您对莫愁个人博客网站平台的认可,非常欢迎各位朋友分享到个人站长或者朋友圈,但转载请说明文章出处“来源莫愁个人博客 https://www.mochoublog.com/study/68.html”。