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sql-serverunique-key
Gomathipriya
Gomathipriya

Reputation: 945

how Unique key works on multiple columns

i have a table T with following columns

col1 col2 col3 col4
1     1   1     1
1     1   1     2
1     1   2     1
1     1   2     1

if i set a column col2,col3,col4 as unique. how does the unique works ? will it take uniqueness of combination of each column?

Upvotes: 0

Views: 357

Answers (3)

MarkD
MarkD

Reputation: 5316

Yes, the "unique-ness" is a result of all columns involved in the constraint. See SO Question You can easily write yourself a table and test how it handles INSERTs

Upvotes: 1

trop
trop

Reputation: 35

I'm not entirely sure, but I think the unique attribute has to do with indexing the table. Whichever column you set as unique, that column should be the one you call on to find a certain row. For example in a call like 

UPDATE table_name SET column_name = some_value WHERE ID = some_number

the ID column should be set to unique, though I don't know whether not doing so would actually stop you from finding a specific row.

Upvotes: 0

akronymn
akronymn

Reputation: 2446

See here: http://www.w3schools.com/sql/sql_unique.asp

The syntax for setting multiple columns as unique is different from that of setting one column unique. If you have multiple columns as unique it is the set that is viewed for uniqueness.

Upvotes: 1

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