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The Keys folder in my table lists the Primary and Foreign keys.There is one other key. What is it?

Posted on 2011-02-25
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Last Modified: 2012-08-14
The Keys folder in my table lists the Primary and Foreign keys.

There is one other key. What is it?

Here is a screen shot
keys-folder.png
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Question by:Mr_Shaw
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by:santoshmotwani
ID: 34979151
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Author Comment

by:Mr_Shaw
ID: 34979201
Why is this not in the constraints folder?
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santoshmotwani earned 2000 total points
ID: 34979275
1 down vote accepted
      

A UNIQUE constraint is also known as a UNIQUE KEY constraint. Basically, there can be multiple KEYs for a table. One is selected (somewhat arbitrarily) to be the PRIMARY KEY for the table. Other keys are created as UNIQUE KEY constraints.

As an implementation detail, a UNIQUE KEY constraint is implemented by placing a UNIQUE index on the same columns in the table. However, it is possible to create UNIQUE indexes on a table (via CREATE INDEX), without creating a UNIQUE constraint.

UNIQUE constraints are similar to PRIMARY KEYs - they can be the target reference for a FOREIGN KEY constraint. A UNIQUE index, by itself, cannot be so referenced.

In SSMS, PRIMARY KEY, UNIQUE, and FOREIGN KEY constraints will always show up under the "Keys" folder for a table. CHECK and DEFAULT constraints will show up under the "Constraints" folder


ref: http://stackoverflow.com/questions/3978680/constraint-is-key-is-index-is-constraint
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Author Closing Comment

by:Mr_Shaw
ID: 34979342
thanks
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