| unique constraint [message #373701] |
Wed, 02 May 2001 07:23  |
adelia
Messages: 45
Registered: April 2001
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Member |
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Why I use a unique constraint instead of using Primary key constraint , to create a table ???
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| Re: unique constraint [message #373703 is a reply to message #373701] |
Wed, 02 May 2001 07:35  |
siddharth
Messages: 16
Registered: May 2001
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Junior Member |
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The diffrence between a unique constraint and a primary key constraint is that unique constraint allows a null value in one row of the table if null is allowed for that column.
A column with primary key cannot contain a null value.
HTH
Siddharth
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| Re: unique constraint [message #373708 is a reply to message #373703] |
Wed, 02 May 2001 08:25 |
jayaram
Messages: 9
Registered: May 2001
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Junior Member |
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Its not just one row with null column, you can have many rows with null column when , unique constraint is assigned to column.
no two nulls are same, NUll <> NULL( <> - not equal to)
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| Re: unique constraint [message #373709 is a reply to message #373703] |
Wed, 02 May 2001 08:26 |
jayaram
Messages: 9
Registered: May 2001
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Junior Member |
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Its not just one row with null column, you can have many rows with null column when , unique constraint is assigned to column.
Two nulls in a column are not same, NUll <> NULL( <> - not equal to)
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