Unique column constraints in SharePoint 2010

In this article we will be seeing how to enforce unique value for a list or library column in SharePoint 2010.

In SharePoint 2010 we can enforce uniqueness on values in a list or library column, effectively creating a primary key. When you create a column in the list or library you could see an option "Enforce Unique Values" where you can enforce uniqueness. When you enforce uniqueness on a Lookup column, the list item in the target list can have only one list item looking up to it from the child list.

Supported Column Types

Single line of text
Choice field (but not multichoice)
Number
Currency
Date/ Time
Lookup (but not multivalve)
Person or Group (but not multivalve)
Title (but not in a document library)

Unsupported Column Types

Multiple lines of text
Hyperlink/Picture
Custom Field Types
Calculated Field
Boolean (yes/no)
Modified by
Modified time
UI version
Created time
Checked out to
Content type ID

I have created one list "cl" which has "Title", "TestCol" and "LookUp Col".

When you make Enforce Unique values to "Yes", you will be getting the following pop up.

I have enforced unique values to "Yes" for TestCol (Single Line of text) and LookUp Col.

The items in the list are shown in the following

When I try to add the same value, I was getting the following error

Note: Comparison is case-insensitive, so the values "Test" and "test" are evaluated as equivalent.