Supposedly easy trigger - sql

Question

I've created a trigger which is to block inserted records with a date already existing in a table.
CREATE TRIGGER [dbo].[SpecialOffers_Insert]
ON [dbo].[SpecialOffers]
FOR INSERT,UPDATE
AS
SET NOCOUNT ON
IF EXISTS (SELECT * FROM inserted WHERE SPO_DateFrom IN (SELECT SPO_DateFrom FROM dbo.SpecialOffers))
BEGIN
RAISERROR('Error. ', 16, 1)
ROLLBACK TRAN
SET NOCOUNT OFF
RETURN
END
SET NOCOUNT OFF
It is added to a table:
CREATE TABLE [dbo].[SpecialOffers](
[SPO_SpoId] [int] IDENTITY(1,1) NOT NULL,
[SPO_DateFrom] [datetime] NOT NULL,
[SPO_DateTo] [datetime] NOT NULL)
The table is empty but when trying to insert such record:
INSERT INTO dbo.SpecialOffers (SPO_DateFrom, SPO_DateTo) VALUES ('2015-01-15','2015-01-15')
I got the Error from the trigger. How should I modify the trigger not to get the error?

Answer 1

If the goal is to block inserted records with date already existing in a table, you don't need a trigger - just create a unique constraint on the date field:
ALTER TABLE [dbo].[SpecialOffers]
ADD CONSTRAINT SpecialOffersUQ UNIQUE (SPO_DateFrom)

Answer 2

If you wanted a trigger to prevent overlaps, why didn't you say so:
CREATE TABLE [dbo].[SpecialOffers](
[SPO_SpoId] [int] IDENTITY(1,1) NOT NULL,
[SPO_DateFrom] [datetime] NOT NULL,
[SPO_DateTo] [datetime] NOT NULL,
constraint CK_SO_NoTimeTravel CHECK (SPO_DateFrom <= SPO_DateTo)
)
GO
CREATE TRIGGER NoOverlaps
on dbo.SpecialOffers
after insert,update
as
set nocount on
if exists (
select *
from dbo.SpecialOffers so1
inner join
dbo.SpecialOffers so2
on
so1.SPO_DateFrom < so2.SPO_DateTo and
so2.SPO_DateFrom < so1.SPO_DateTo and
so1.SPO_SpoId != so2.SPO_SpoId
inner join
inserted i
on
so1.SPO_SpoId = i.SPO_SpoId
)
begin
RAISERROR('No overlaps',16,1)
ROLLBACK
end
Examples:
--Works
INSERT INTO SpecialOffers (SPO_DateFrom,SPO_DateTo)
values ('20010101','20011231')
GO
--Fails (Trigger)
INSERT INTO SpecialOffers (SPO_DateFrom,SPO_DateTo)
values ('20010101','20011231')
GO
--Fails (Constraint)
INSERT INTO SpecialOffers (SPO_DateFrom,SPO_DateTo)
values ('20011231','20010101')
GO
--Fails (Trigger)
INSERT INTO SpecialOffers (SPO_DateFrom,SPO_DateTo)
values ('20020101','20021231'),
('20020701','20030630')
I also added a check constraint so that I didn't have to deal with nonsense data in the trigger.
You might have to change swap some of the <s for <=s or vice-versa, depending on what definition of intervals you want to use (i.e. are DateFrom and DateTo meant to be inclusive or exclusive endpoints for the interval they're describing?)

Answer 3

Since the trigger runs in the transaction context of the SQL statement that fired it, after this INSERT, there will be a row in your table dbo.SpecialOffers with the SPO_DateFrom values you've just inserted and the SELECT from the table will succeed ...
Therefore, the trigger will assume that there's already been a value - and it throws the error (as designed).
You could rewrite the trigger to not look at the newly inserted rows, but anything else - but as others have pointed out, a UNIQUE constraint does that much more simply

Answer 4

You should check if the rows you found are actually NOT the ones you have just inserted. Change the line
IF EXISTS (
SELECT * FROM inserted
WHERE SPO_DateFrom IN (
SELECT SPO_DateFrom
FROM dbo.SpecialOffers)
)
To
IF EXISTS (
SELECT * FROM inserted a
WHERE SPO_DateFrom IN (
SELECT SPO_DateFrom
FROM dbo.SpecialOffers b
WHERE a.SPO_SpoId <> b.SPO_SpoId)
)