How to use the original value in a SQL Server update trigger to change a second record - sql

Question

I have a SQL Server table with student, cohort, program, grad date, and status. Some students have records that are specifically tied together – if the cohort for one row changes, the cohort for another row with status 2 needs to change as well. In most cases I catch this programmatically but there are a LOT of ways to change a student’s cohort so we’re missing some.
Essentially if someone runs the first line update statement, the second also needs to run
UPDATE StudentPrograms
SET Cohort = 201610
WHERE Student = 'A1234' AND Program = 'MBA' AND Cohort = 201510
UPDATE StudentPrograms
SET Cohort = 201610
WHERE Student = 'A1234' AND Cohort = 201510 AND Status = 2
I would like to put an update trigger on the table to change both cohorts, but the problem is I can’t figure out how to capture both the starting cohort that identifies the record that needs to change and the cohort that it would need to change to. Once the update has run and the trigger fires, all I have is the updated cohort, so I don’t know which record to change.
CREATE TRIGGER [dbo].[tr_SP_Cohort]
ON [dbo].StudentPrograms
AFTER UPDATE
AS
DECLARE #student VARCHAR(10);
DECLARE #cohort INT;
SELECT #student = i.student FROM inserted i
SELECT #cohort = i.cohort FROM inserted i
SET NOCOUNT ON;
IF UPDATE(Cohort)
UPDATE dbo.StudentPrograms
SET Cohort = #cohort
WHERE Student = #student AND Status = 2 AND Cohort = ???
Unfortunately the records do have to be in this table and not a sub table for a lot of reasons. The records are occasionally also deleted / reinserted, so I run into the same problems if I tie them together by adding another field with a key - if that record gets deleted / updated / reinserted, I still need to capture the information, record it, then record the change.
Thanks for the immediate help! Here's what I ended up with, changed a bit so it works with batch updates.
ALTER TRIGGER [dbo].[tr_SP_Cohort]
ON [dbo].StudentPrograms
AFTER UPDATE
AS
SET NOCOUNT ON;
IF UPDATE(Cohort)
UPDATE a
SET Cohort = i.Cohort
FROM dbo.StudentPrograms AS a
INNER JOIN inserted AS i ON a.Student = i.Student
INNER JOIN deleted AS d ON a.Cohort = d.Cohort
AND a.Student = d.Student
WHERE a.Status = 2

Answer 1

When creating a trigger, you can access the old values using the deleted table
WHERE Student = #student AND Status = 2 AND Cohort = (SELECT TOP 1 Cohort from deleted)

Answer 2

Ideally, you should handle this in a stored procedure but as a temporary fix you can use the following code.
They way you are handling updates in your code, it will never work if more than one row was updated.
Triggers are fired once in response to a triggering action (update,delete,insert) for all the rows affected by that action, NOT for each row affected by the triggering action.
Keeping all this in mind your temporary solution should look something like...
CREATE TRIGGER [dbo].[tr_SP_Cohort]
ON [dbo].StudentPrograms
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
UPDATE s
SET s.Cohort = i.Cohort
FROM deleted d
INNER JOIN dbo.StudentPrograms s ON d.Student = s.Student
AND d.Cohort = s.Cohort
INNER JOIN inserted i ON i.Student = s.Student
WHERE s.[Status] = 2;
END