您首先做错了事，然后尝试对其进行修复。那是行不通的。
您想加入的事物是
select * from stuff where grp = 'a'和
select * from stuff where grp ='b'。因此，加入这些：
select a.ID as a, b.ID as b from (select * from stuff where grp = 'a') afull join (select * from stuff where grp = 'b') bon b.id = a.id
SQL小提琴
