LEFT JOIN具有特定条件

Question

I have two tables

table A
---------
uid   c1(distinct)
---------
 1     a
 1     b
 2     c
 3     d
 3     e
 3     f
 4     g

and

table B
---------
uid   c2(non unique)
---------
 1     x
 2     y
 3     z
 3     z  
 3     v

I need a resultant table which maps table A to table B on uid like this

Result Table
-------------
uid   c1   c2
-------------
 1     a    x
 1     b    -
 2     c    y
 3     d    z
 3     e    z
 3     f    v
 4     g    -

Basically once a row from table B is used in the resultant table, it must be exhausted. None of the JOINS work in this case.

Both these tables contain large number of records, so I can't write individual queries for each uid.

Answer 1

Let's count the rows with the same uid and join equal uids and that numbers

select t.uid, c1, ifnull(c2, '-') c2 
    from 
      (select *, @n:=if(@i=uid, @n+1, if(@i:=uid, 1, 1)) n 
           from tableA 
               cross join 
                (select @n:=0, @i:='') param1 
          order by uid
      ) t 
    left join 
      (select *, @n1:=if(@i1=uid, @n1+1, if(@i1:=uid, 1, 1)) n1 
           from tableB 
               cross join 
                (select @n1:=0, @i1:='') param2 
         order by uid
      ) tt 
          on t.uid=tt.uid and t.n=tt.n