[snip]
I have a query that I have in mind, but am not sure of how I can
actually write it. It might not even be possible. I was hoping someone
could tell me if I will have to use two queries instead, or if this will
actually work:
(In simplified form:)
+--------+
+-------+ | main |
| sub1 | +--------+ +-------+
+-------+ | id | | sub2 |
| id |---<| sub1fk | +-------+
| other | | sub2fk |>---| id |
+-------+ +--------+ | other |
+-------+
As you can see from the simple diagram, I have a main table with its own
primary key (id) but with two foreign key columns. The first one
(sub1fk) points to the primary key of the table "sub1". The second one
(sub2fk) points to the primary ky of the table "sub2".
.....
+----+-----------+-----------+
| id | sub1other | sub2other |
+----+-----------+-----------+
| 1 | 2 | NULL |
| 2 | NULL | 5 |
| 3 | NULL | 17 |
| 4 | 8 | NULL |
| .. | ...etc | ...etc |
+----+-----------+-----------+
Later, in my application, I can test each column for NULL and I will
know that the other column is the one to use (for instance, if the value
of the "sub1other" column is NULL in one record, then I'll use the value
of sub2other to do what I want to do, and vice versa).
[/snip]
Erik,
I don't have time to go into detail, but look into LEFT OUTER JOIN and RIGHT
OUTER JOIN. You may be able to accomplish what you are trying to using
these. I have done similar things in the past and was able to get the
desired results.
HTH!
Jay
sql, mysql, query
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