Question

We have a user table, every user has an unique email and username. We try to do this within our code but we want to be sure users are never inserted (or updated) in the database with the same username of email. I’ve added a BEFORE INSERT Trigger which prevents the insertion of duplicate users.

CREATE TRIGGER [dbo].[BeforeUpdateUser]    ON  [dbo].[Users]    INSTEAD OF INSERT AS  BEGIN     -- SET NOCOUNT ON added to prevent extra result sets from     -- interfering with SELECT statements.     SET NOCOUNT ON;      DECLARE @Email nvarchar(MAX)     DECLARE @UserName nvarchar(MAX)     DECLARE @UserId int     DECLARE @DoInsert bit      SET @DoInsert = 1      SELECT @Email = Email, @UserName = UserName FROM INSERTED      SELECT @UserId = UserId FROM Users WHERE Email = @Email      IF (@UserId IS NOT NULL)         BEGIN             SET @DoInsert = 0         END      SELECT @UserId = UserId FROM Users WHERE UserName = @UserName      IF (@UserId IS NOT NULL)         BEGIN             SET @DoInsert = 0         END      IF (@DoInsert = 1)         BEGIN             INSERT INTO Users             SELECT                             FirstName,                             LastName,                             Email,                             Password,                             UserName,                             LanguageId,                            Data,                             IsDeleted                         FROM INSERTED         END     ELSE         BEGIN             DECLARE @ErrorMessage nvarchar(MAX)             SET @ErrorMessage =                           'The username and emailadress of a user must be unique!'             RAISERROR 50001 @ErrorMessage         END  END 

But for the Update trigger I have no Idea how to do this. I’ve found this example with google: http://www.devarticles.com/c/a/SQL-Server/Using-Triggers-In-MS-SQL-Server/2/ But I don’t know if it applies when you update multiple columns at once.

EDIT:

I’ve tried to add a unique constraint on these columns but it doesn’t work:

Msg 1919, Level 16, State 1, Line 1 Column 'Email' in table 'Users' is of a type  that is invalid for use as a key column in an index. 

Answer 1

You can add a unique contraint on the table, this will raise an error if you try and insert or update and create duplicates

ALTER TABLE [Users] ADD  CONSTRAINT [IX_UniqueUserEmail] UNIQUE NONCLUSTERED  (     [Email] ASC )  ALTER TABLE [Users] ADD  CONSTRAINT [IX_UniqueUserName] UNIQUE NONCLUSTERED  (     [UserName] ASC ) 

EDIT: Ok, i’ve just read your comments to another post and seen that you’re using NVARCHAR(MAX) as your data type. Is there a reason why you might want more than 4000 characters for an email address or username? This is where your problem lies. If you reduce this to NVARCHAR(250) or thereabouts then you can use a unique index.