adding primary keys

would adding primary keys to some of the live data tables i have on my MSSQl database change anything to the data.  WOuld it affect the data structure or anything?
shpresaAsked:
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Daniel WilsonCommented:
No, it would not change the data.
If the data would violate that primary key, that is, the value is not unique, the creation of the primary key will fail.
It does change the structure because that PK is part of the structure.  It will not add fields, change data types, or any of that sort of structure change.
If you application already inserts only data that are unique on the fields in question, there will be no problem caused
If your application inserts "empty" rows, then updates them, adding primary keys may cause problems.

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Jim HornMicrosoft SQL Server Data DudeCommented:
For starters, why is there 'live data tables' without primary keys?  

If your table has column(s) that uniquely identify the row, then they should be defined as a primary key, as the use of primary keys and foreign key relationships greatly improve query performance vs. a table without a primary key (also known as a 'heap').

If you're at this point now then I highly recommend having an experienced data architect review your database and make recommendations.
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