Question

I wish to make sure that my data has a constraint the following check (constraint?) in place

• This table can only have one BorderColour per hub/category. (eg. #FFAABB)
• But it can have multiple nulls. (all the other rows are nulls, for this field)

Table Schema

ArticleId INT PRIMARY KEY NOT NULL IDENTITY
HubId TINYINT NOT NULL
CategoryId INT NOT NULL
Title NVARCHAR(100) NOT NULL
Content NVARCHAR(MAX) NOT NULL
BorderColour VARCHAR(7) -- Can be nullable.

I’m gussing I would have to make a check constraint? But i’m not sure how, etc.

sample data.

1, 1, 1, 'test', 'blah...', '#FFAACC'
1, 1, 1, 'test2', 'sfsd', NULL
1, 1, 2, 'Test3', 'sdfsd dsf s', NULL
1, 1, 2, 'Test4', 'sfsdsss', '#AABBCC'

now .. if i add the following line, i should get some sql error….

INSERT INTO tblArticle VALUES (1, 2, 'aaa', 'bbb', '#ABABAB')

any ideas?

Answer 1

Another option that is available is available if you are running SQL2008. This version of SQL has a feature called filtered indexes.

Using this feature you can create a unique index that includes all rows except those where BorderColour is null.

CREATE TABLE [dbo].[UniqueExceptNulls](
    [HubId] [tinyint] NOT NULL,
    [CategoryId] [int] NOT NULL,
    [BorderColour] [varchar](7) NULL,
)

GO

CREATE UNIQUE NONCLUSTERED  INDEX UI_UniqueExceptNulls
ON [UniqueExceptNulls] (HubID,CategoryID)
WHERE BorderColour IS NOT NULL

This approach is cleaner than the approach in my other answer because it doesn’t require creating extra computed columns. It also doesn’t require you to have a unique column in the table, although you should have that anyway.

Finally, it will also be much faster than the UDF/Check Constraint solutions.