can i consider 3 columns of a single table together as a composite primary key.... & wat if i want to refer just 1 column out of that in some other table as a foreign key....is it possible....plzzz reply as soon as possible...thank u..
+1
A:
yes you can put 3 columns as primary key, also you can put one of them as foreign key.
Wael Dalloul
2009-09-03 06:54:48
A:
Yes, on both accounts. 3 columns can be used as a composite key and 1 of those could be used as a foreign key.
Taylor Leese
2009-09-03 06:55:13
You've got to be able to take into account the fact that the columns you are using might not be unique.
Mauro
2009-09-03 07:27:04
I was assuming the combination of the 3 columns would be unique since he wants to use it as a primary key. Otherwise, the question doesn't really make sense.
Taylor Leese
2009-09-03 18:04:53
A:
Foreign keys must reference a set of columns over which a unique constraint is defined (SQL 2008 standard 11.8: Syntax Rules: 3.a). A single attribute of a composite primary key does not necessarily have a unique constraint defined.
If the single attribute does have a unique constraint defined, that most likely indicates bad design -- the primary key's unique constraint would be redundant.
With that in mind, I would say that "no" is the correct answer to your question, although I suppose technically it may be possible in some situations that I doubt apply in your case.
Jeff Davis
2009-09-03 15:11:34