How to fire trigger in SQL Server if in a same row 2 column value are same?

Question

How to fire trigger in SQL Server if in a row 2 column values are the same?

In my table there are 3 columns id , capacity , taken and the table has some rows. If taken is equal to capacity , then trigger should fire. I wrote the trigger shown below, but it says "subquery returned more than 1 value" - this is not permitted.

ALTER TRIGGER [dbo].[sectionTrigger1]
ON [dbo].[tblTestCapacity]
FOR UPDATE 
AS
BEGIN
    SET NOCOUNT ON

    IF (SELECT (Capacity-SeatTaken) FROM tblTestCapacity) = 0
    BEGIN
        INSERT INTO tblTest(sectionid, courseid, seatcapacity, seattaken)
            SELECT sectionid, courseid, seatcapacity, seattaken 
            FROM tblTest 
    END
END

Answer 1

inside a trigger you should use the inserted table, which will hold all updated rows

something like this maybe ?

INSERT INTO tblTest(sectionid, courseid, seatcapacity, seattaken)
SELECT i.sectionid, i.courseid, i.seatcapacity, i.seattaken 
FROM   inserted i
where  (i.Capacity - i.SeatTaken) = 0

notice you dont need the IF anymore because it is already in the WHERE clause

EDIT
IF you want to do something else when (i.Capacity - i.SeatTaken) = 0 you can use the same query

if exists ( SELECT i.sectionid, i.courseid, i.seatcapacity, i.seattaken 
            FROM   inserted i
            where  (i.Capacity - i.SeatTaken) = 0
          )
begin
     -- do here your code if any row exists with this condition
     -- you could for example throw an exception here
     THROW 51000, 'your message here.', 1;
end

EDIT
if your goal is to restrict 2 columns in the table from having the same value, you can also add a constraint.
This way you dont even need a trigger
This is the best option, if all you need is prevent 2 columns from having the same value

alter table tblTestCapacity
add constraint CH_Doubles check (Capacity <> SeatTaken)

if both or one column can be null, you have to add a little more

alter table tblTestCapacity
add constraint CH_Doubles check (isnull(Capacity, -1) <> isnull(SeatTaken, -1))

The last alter statement is not tested, not sure if sql-server allows this