select a.code 
from a join (select b.code from b,c,d where b.code=c.code and b.code=d.code) t 
       on a.code = t.code

------解决方案--------------------
SQL code

select distinct a.code from a,b,c,d where a.code=b.code and b.code=c.code and b.code=d.code

------解决方案--------------------
SQL code

select a.*
from a
inner join b on a.code=b.code
inner join c on a.code=c.code
inner join d on a.code=d.code

------解决方案--------------------
探讨


SQL code

select a.*
from a
inner join b on a.code=b.code
inner join c on a.code=c.code
inner join d on a.code=d.code