SQL Server 2005 How Create a Unique Constraint?

The SQL command is:

ALTER TABLE <tablename> ADD CONSTRAINT
            <constraintname> UNIQUE NONCLUSTERED
    (
                <columnname>
    )

See the full syntax here.

If you want to do it from a Database Diagram:

• right-click on the table and select 'Indexes/Keys'
• click the Add button to add a new index
• enter the necessary info in the Properties on the right hand side:
  • the columns you want (click the ellipsis button to select)
  • set Is Unique to Yes
  • give it an appropriate name

ALTER TABLE [TableName] ADD CONSTRAINT  [constraintName] UNIQUE ([columns])

Warning: Only one null row can be in the column you've set to be unique.

You can do this with a filtered index in SQL 2008:

CREATE UNIQUE NONCLUSTERED INDEX idx_col1
ON dbo.MyTable(col1)
WHERE col1 IS NOT NULL;

See Field value must be unique unless it is NULL for a range of answers.

ALTER TABLE dbo.<tablename> ADD CONSTRAINT
            <namingconventionconstraint> UNIQUE NONCLUSTERED
    (
                <columnname>
    ) ON [PRIMARY]

I also found you can do this via, the database diagrams.

By right clicking the table and selecting Indexes/Keys...

Click the 'Add' button, and change the columns to the column(s) you wish make unique.

Change Is Unique to Yes.

Click close and save the diagram, and it will add it to the table.

You are looking for something like the following

ALTER TABLE dbo.doc_exz
ADD CONSTRAINT col_b_def
UNIQUE column_b

MSDN Docs

In the management studio diagram choose the table, right click to add new column if desired, right-click on the column and choose "Check Constraints", there you can add one.