How can I create a SQL unique constraint based on 2 columns?

Question

I have a Table like this one:

|UserId   |  ContactID |  ContactName 
---------------------------------------
| 12456   |  Ax759     |  Joe Smith
| 12456   |          


        

Answer 1

You can try this:

CREATE UNIQUE CLUSTERED INDEX index_name ON TABLE (col1,col2)
or
CREATE UNIQUE NONCLUSTERED INDEX index_name ON TABLE (col1,col2)

or

ALTER TABLE [dbo].[TABLE] ADD CONSTRAINT
UNIQUE_Table UNIQUE CLUSTERED
(
col1,
col2
) ON [PRIMARY]

Answer 2

You can try ALTER TABLE [TABLE_NAME] ADD UNIQUE (column1,column2,column3 ...columnN).

Hope this helps cheers.

Answer 3

You can add unique constraint tou your fields:

ALTER TABLE YourTable
ADD CONSTRAINT UQ_UserId_ContactID UNIQUE(UserId, ContactID)

Answer 4

CREATE TABLE [LineItems](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [OrderID] [int] NOT NULL,
    [LineItemNumber] [int] NOT NULL,
 CONSTRAINT [PK_LineItems] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
),
 CONSTRAINT [UC_LineItems] UNIQUE NONCLUSTERED 
(
    [OrderID] ASC,
    [LineItemNumber] ASC
)
)

Answer 5

Here is the syntax for creating a unique CONSTRAINT as opposed to a unique INDEX.

ALTER TABLE publishers 
  ADD CONSTRAINT uqc_pub_name 
  UNIQUE (pub_name)

It is important to note that there are subtle differences dependent on which method you use to enfore the uniqueness of a column.

See the following MSDN reference for an interesting walkthrough of these:

http://msdn.microsoft.com/en-us/library/aa224827(SQL.80).aspx