MySQL数据库中分组统计的问题_MySQL

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A　 |　B　 |　C　 |　D 
a b | a c | a d | d e 
1 1 | 1 1 | 1 A | A 1 
2 2 | 1 3 | 2 B | B 2 
3 4 | 1 4 | 3 C | C 3 
4 6 | 2 1 | 4 D | D 4 
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1 1 3 1 
2 2 1 2 
3 4 0 3 
4 6 0 4

select A.a,A.b,IFNULL(c,0) as c,D.a 
from (select 1 as a,2 as b union all
select 2 as a,2 as b union all
select 3 as a,4 as b union all
select 4 as a,6 as b) as A 
left outer join 
(select a,count(a) as c from(
select 1 as a,3 as c union all
select 1 as a,3 as c union all
select 1 as a,3 as c union all
select 2 as a,3 as c ) B group by a) as B
on A.a=B.a 
join 
(select 1 as a,'A' as d union all
select 2 as a,'B' as d union all
select 3 as a,'C' as d union all
select 4 as a,'D' as d ) as C 
on A.a=C.a 
join(select 1 as a,'A' as d union all
select 2 as a,'B' as d union all
select 3 as a,'C' as d union all
select 4 as a,'D' as d ) as D
on D.d=C.d