Re: Join A to B via AB where AB may or may not join

On Dec 15, 5:03 am, "cc" <chris.colclo..._at_nospam.jhuapl.edu> wrote:
> SELECT *
>
> FROM TABLE_AB AB
>
> WHERE EXISTS (
>
> SELECT AB.A
>
> FROM TABLE_A
>
> WHERE AB.A = TABLE_A.A)
>
> UNION
>
> SELECT A, B
>
> FROM TABLE_A, TABLE_B
>
> WHERE NOT EXISTS (
>
> SELECT A, B
>
> FROM TABLE_AB
>
> WHERE TABLE_A.A = TABLE_AB.A)
>
> ORDER BY 1,2
>

Another solution may be as following.(Not tested.) SELECT A.a

     , COALESCE(AB.b, B.b) AS b
  FROM Table_A A

       LEFT OUTER JOIN
       Table_AB AB
         ON AB.a = A.a
       LEFT OUTER JOIN
       Table_B  B
         ON AB.a IS NULL;