Question

I would like to know whether I could perform a full outer join between two tables on a full column list first, and if this fails, a partial column list.
For example:

Table A
|  a  |  b  |  c  |
+-----+-----+-----+
|  x  |  y  |  z  |
|  q  |  r  |  s  |


Table B
|  a  |  b  |  c  |  d  |  e  |
+-----+-----+-----+-----+-----+
|  q  |  r  |  s  |  t  |  u  |
|  q  |  r  |     |  x  |  y  |
|     |     |     |  x  |  y  |

Failed query:
SELECT *
FROM A FULL OUTER JOIN B
ON
 (A.a = B.a AND A.b = B.b AND A.c = B.c)
 OR (A.a = B.a AND A.b = B.b)

Desired result
|  a  |  b  |  c  |  d  |  e  |
+-----+-----+-----+-----+-----+
|  x  |  y  |  z  |     |     |
|  q  |  r  |  s  |  t  |  u  |
|  q  |  r  |     |  x  |  y  |
|     |     |     |  x  |  y  |

I am using Postgresql. Thanks for your assistance.

Answer 1

This query would produce the desired result exactly:

SELECT *
FROM   A
FULL   JOIN B USING (a,b,c)
ORDER  BY a DESC NULLS LAST;

But I am not entirely convinced that your example fits your description – which leaves room for interpretation.