Question

假设我有一张几乎没有字段的表，例如A，B，C，D

我需要按字段A分组，并从B，C，D中选择最常出现的值。

示例：

+---+---+---+----+
| A | B | C | D  |
+---+---+---+----+
| 1 | 3 | 5 | 15 |
+---+---+---+----+
| 1 | 5 | 6 | 32 |
+---+---+---+----+
| 1 | 5 | 6 | 34 |
+---+---+---+----+
| 2 | 7 | 5 | 50 |
+---+---+---+----+
| 2 | 8 | 1 | 32 |
+---+---+---+----+

预期结果：

+---+---+---+----+
| A | B | C | D  |
+---+---+---+----+
| 1 | 5 | 6 | 15 |
+---+---+---+----+
| 2 | 7 | 5 | 50 |
+---+---+---+----+

我看到了很多例子，如何使用COUNT（*）从一列中选择大多数出现的值，而不是在其上使用MAX。但在这种情况下该怎么办？

Answer 1

查询看起来有点复杂，因为您必须为3列执行此操作。我们的想法是按照a-b，a-c，a-d的组合对每个组合的第一行进行分组，对行进行排名。这是使用变量完成的。在计数关联的情况下，返回b，c或d的最低值。（如果需要颠倒排序，则可以更改。）最后需要再一次聚合才能将相应的值传递到一行。

SQL Fiddle Demo

select a,max(b),max(c),max(d)
from (
select a,b,c,d 
from (select a,b,c,d,
      @rn:=case when @prev=a then @rn+1 else 1 end as rank,
      @prev:=a
      from (select a,b,null as c,null as d,count(*) as cnt
            from tbl
            group by a,b
           ) t
      cross join (select @rn:=0,@prev:='') r
      order by a,cnt desc,b
     ) t
where rank = 1
union all
select a,b,c,d 
from (select a,b,c,d,
      @rn:=case when @prev=a then @rn+1 else 1 end as rank,
      @prev:=a
      from (select a,null as b,c,null as d,count(*) as cnt
            from tbl
            group by a,c
           ) t
      cross join (select @rn:=0,@prev:='') r
      order by a,cnt desc,c
     ) t
where rank = 1
union all
select a,b,c,d
from (select a,b,c,d,
      @rn:=case when @prev=a then @rn+1 else 1 end as rank,
      @prev:=a
      from (select a,null as b,null as c,d,count(*) as cnt
            from tbl
            group by a,d
           ) t
      cross join (select @rn:=0,@prev:='') r
      order by a,cnt desc,d
     ) t
where rank = 1
) t
group by a

在MYSQL中的GROUP BY之后，在几个字段中选择大多数出现的值

1 个答案: