Trigger to prevent Insertion for duplicate data of two columns

Question

I am working on SQL Server 2008R2, I am having the following Table

ID     Name     date
 1     XYZ      2010
 2     ABC      2011
 3     VBL      2010

Now i want to prevent insertion if i have a Data although the ID is different but data is present

 ID    Name     date
  4    ABC      2011

Kindly guide me how should i write this trigger.

Answer 1

Something like this:

CREATE TRIGGER MyTrigger ON dbo.MyTable
AFTER INSERT
AS

if exists ( select * from table t 
    inner join inserted i on i.name=t.name and i.date=t.date and i.id <> t.id)
begin
    rollback
    RAISERROR ('Duplicate Data', 16, 1);
end
go

That's just for insert, you might want to consider updates too.

Update

A simpler way would be to just create a unique constraint on the table, this will also enforce it for updates too and remove the need for a trigger. Just do:

ALTER TABLE [dbo].[TableName]    
ADD CONSTRAINT [UQ_ID_Name_Date] UNIQUE NONCLUSTERED
(
    [Name], [Date]
)

and then you'll be in business.

Answer 2

If you are using a store procedure inserting data into the table, you don't really need a trigger. You first check if the combination exists then don't insert.

CREATE PROCEDURE usp_InsertData
@Name varchar(50),
@Date DateTime
AS
BEGIN

IF (SELECT COUNT(*) FROM tblData WHERE Name = @Name AND Date=@Date) = 0
    BEGIN
        INSERT INTO tblData
                    ( Name, Date)
             VALUES (@Name, @Date)
        Print 'Data now added.'
     END
ELSE
    BEGIN
        Print 'Dah! already exists';
    END
END

The below trigger can used if you are not inserting data via the store procedure.

CREATE TRIGGER checkDuplicate ON tblData
AFTER INSERT
AS

IF EXISTS ( SELECT * FROM tblData A 
INNER JOIN inserted B ON B.name=A.name and A.Date=B.Date)
BEGIN
    RAISERROR ('Dah! already exists', 16, 1);
END
GO