| unique constraint [message #38392] |
Tue, 16 April 2002 07:59  |
Rolf
Messages: 2
Registered: April 2002
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Junior Member |
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Hi all,
who can answer this exam-question?
Table TEST has a unique constraint on the two nullable columns a and b. How many rows can contain a='TEST' and NULL b?
A) 0
B) 1
C) Any number up to all rows -1
D) Any number up to all rows
Please explain your choice. thanx
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| Re: unique constraint [message #38400 is a reply to message #38392] |
Tue, 16 April 2002 17:35  |
Todd Barry
Messages: 4819
Registered: August 2001
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Senior Member |
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There can only be 1 row with those values. A unique constraint, unlike a primary key constraint allows NULL values, but any combination of values/NULLs in the key can only appear once.
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