How can I force a column to be unique for an entire table in SQL Server 2008 R2?

Question

I have a table with a Description field. I want to ensure that no two rows have the same "Description," but I can't make Description into my identity column (my ID column is an int).

Is it safe to set Description as a second primary key (in addition to my ID which is already a primary key)?

Answer 1

There is no such thing as a 'secondary primary key'. There is one primary key per table.

Create a UNIQUE constraint on the Description column (highly unusual thing to do, BTW. For example, It is more usual to create a unique index on Product Name rather than a Product description ) or if you have null values in the Description column create a Filtered index (SQL Server 2008 onwards)

ALTER TABLE dbo.yourTable
   ADD CONSTRAINT UQ_yourTable_Description UNIQUE ([Description]);

Answer 2

Add a Unique index to the Description column.

Using Sql Server Management Studio right click on the table and choose Design. Then right click on a column and choose "Indexes/keys". You will be prompted with the following window

Click on Add on the bottom left and then specify properties for your index. If you want to use a DDL script then use something like this

CREATE UNIQUE NONCLUSTERED INDEX [IX_INDEXNAME] ON [dbo].[TABLENAME] 
(
    [Description] ASC
)
WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
GO

Answer 3

There is another way to do this with the SSMS GUI if you prefer:

I've replaced the Description column with the name MyUniqueColumn for the sake of the example.

1. Right click "Indexes" under your table in the SSMS Solution Explorer and click "New Index..." (I know you are looking to create a contstraint, not an index, but this is exactly what the ADD CONSTRAINT SQL script does.

2. Give new index a name (eg "UQ_MyUniqueColumn"), check "Unique", and click "Add..."

3. Check your column in the next window

4. Click OK in both windows