mysql - 在 MYSQL 中，GROUP BY 之后在几个字段中选择最常出现的值

Question

假设我有一个包含几个字段的表，例如A、B、C、D

我需要按字段 A 进行分组，并从 B、C、D 中选择出现次数最多的值。

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 |
+---+---+---+----+

我看到了很多示例，如何使用 COUNT(*) 从一列中选择出现次数最多的值，而不是在此基础上使用 MAX。但在这种情况下该怎么办？

Answer 1

该查询看起来有点复杂，因为您必须对 3 列执行此操作。这个想法是通过按 a-b、a-c、a-d 的组合分组的计数对行进行排名，并获取每个组合的第一行。这是使用变量完成的。如果计数相同，则返回 b、c 或 d 的最小值。 (如果需要颠倒顺序，则可以更改此设置。)最后，还需要进行一次聚合才能将相应的值获取到一行。

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