Primary key vs Unique key

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Primary key vs Unique key

RKaram

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October 27, 2014 at 11:02 pm

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rkaram
October 28, 2014 at 4:02 am

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Vimal Lohani SSCommitted Points: 1650 More actions · Answer 1

+1

Vimal LohaniSQL DBA | MCP (70-461,70-462)==============================The greatest barrier to success is the fear of failure ** Success is a journey not a destination**Think before you print, SAVE TREES, Protect Mother Nature

Hany Helmy SSChampion Points: 13921 More actions · Answer 2

Eaaazy, I think there was a similar question b4.

Have a nice day 🙂

Thanks & Best Regards,
Hany Helmy
SQL Server Database Consultant

pmadhavapeddi22 SSCertifiable Points: 5306 More actions · Answer 3

Hany Helmy (10/27/2014)
Eaaazy, I think there was a similar question b4.
Have a nice day 🙂

+1

serg-52 SSCrazy Eights Points: 9913 More actions · Answer 4

"only one null " is someway misleading. Precisley it's a "only one null value per column".

Koen Verbeeck SSC Guru Points: 259115 More actions · Answer 5

Easy question, thanks!

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Koen Verbeeck SSC Guru Points: 259115 More actions · Answer 6

serg-52 (10/28/2014)
"only one null " is someway misleading. Precisley it's a "only one null value per column".

If the unique index is not a filtered index 😉

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Emil B SSCertifiable Points: 5659 More actions · Answer 7

Question that is very often asked on interviews 🙂

Thanks!

Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 8

Koen Verbeeck (10/28/2014)
serg-52 (10/28/2014)
"only one null " is someway misleading. Precisley it's a "only one null value per column".
If the unique index is not a filtered index 😉

The question was about a unique "key", which I assume is intended to mean unique "constraint" not "index" (given the context with primary key in the same sentence, and the answer option about defauult index created).

One NULL is correct for a single-column constraint, not for composite. A better way to phrase it is that a UNIQUE constraint allows NULLs (as per ANSI standard), but treats NULL as a normal value that also has to conform to the uniqueness constraint (in violation of that same standard).

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
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serg-52 SSCrazy Eights Points: 9913 More actions · Answer 9

Koen Verbeeck (10/28/2014)
serg-52 (10/28/2014)
"only one null " is someway misleading. Precisley it's a "only one null value per column".
If the unique index is not a filtered index 😉

It's another point of confusion. SQL has PRIMARY KEY but has no any 'UNIQUE KEY'. We have UNIQUE constraint and UNIQUE INDEX instead. Most probably 'UNIQUE KEY' here is supposed to be UNIQUE constraint which has no WHERE (filtering) option.