在SQL左外部联接中，如何仅联接首次出现？

Question

Consider the following tables:

Table A

Category  Misc
--------  ----
a         fsd
a         wer
b         fgg
c         ghj 
c         yui
c         rty

Table B (maps category to a number)

Category  Num
--------  ---
a         1
b         2
c         3

Desired Outcome

Category  Num  Misc
--------  ---  ----
a         1    fsd
a              wer
b         2    fgg
c         3    ghj
c              yui
c              rty

A simple left outer join will try to map Num to every Category in Table A, but I only want Num to appear once. Can this be done?

Answer 1

I'm still confused about the reason, but with your modification it is quite easily possible. By left joining the same table grabbing any (the lowest in this case) Misc column, you can get a record for only one of the occurrences of each Cateogory in TableA. Then you can left join TableB only on those rows where a record was found in that grouped subquery.

select
  a.Category,
  a.Misc,
  b.Num
from
  TableA a
  left join (
    select 
      Category, 
      min(Misc) as MinMisc 
    from 
      TableA
    group by
      Category) c on c.Category = a.Category and c.MinMisc = a.Misc
  left join TableB b on b.Category = a.Category and mc.Category is not null