Trigger to update parent table based on max value of child table - tsql

Question

this is my first foray into SSMS/T-SQL (coming from Access). I have a trigger setup that keeps the value of a column in a parent table always equal to the MAX value of a column a child table based on the key between them. To calculate the MAX I have a UDF defined that i Think works ok.
The problem I seem to have is that the trigger executes for EVERY key in the table and not just the one that got updated/deleted/inserted (or so is what I can glean from the debugger).
Here is the parent table:
CREATE TABLE [dbo].[factMeasures](
[MeasureID] [int] IDENTITY(1,1) NOT NULL,
[QARTOD] [int] NULL,
[Param] [char](10) NOT NULL,
[Value] [real] NOT NULL,
CONSTRAINT [PK_factMeasures] PRIMARY KEY CLUSTERED
(
[MeasureID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
Here is the child table:
CREATE TABLE [dbo].[dt_QCflags](
[QC_ID] [int] IDENTITY(1,1) NOT NULL,
[fkMeasureID] [int] NOT NULL,
[RuleValue] [int] NOT NULL,
CONSTRAINT [PK_dt_QCflags] PRIMARY KEY CLUSTERED
(
[QC_ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[dt_QCflags] WITH CHECK ADD CONSTRAINT [FK_dt_QCflags_factMeasures] FOREIGN KEY([fkMeasureID])
REFERENCES [dbo].[factMeasures] ([MeasureID])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[dt_QCflags] CHECK CONSTRAINT [FK_dt_QCflags_factMeasures]
GO
HEre is the UDF that calculates the MAX value of [RuleValue] for the input [MeasureID]
CREATE FUNCTION [dbo].[MaxQC](#MeasureID INT)
RETURNS INT
AS
BEGIN
RETURN
(SELECT
Max([dt_QCflags].[RuleValue]) AS Max_RuleValue
FROM
dbo.dt_QCflags
WHERE
dt_QCflags.fkMeasureID = #MeasureID
GROUP BY
fkMeasureID);
END
And here is the trigger on the child table:
ALTER TRIGGER [dbo].[UpdateQARTOD]
ON [dbo].[dt_QCflags]
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
UPDATE factMeasures
SET QARTOD = dbo.MaxQC(MeasureID) -- by QARTOD Definition, QARTOD flag is set to the MAX of all sub-test results
END
So what I want is for the column in the parent (factMeasures.QARTOD) to always contain the Maximum of the column in the child table (dt_QCFlags.RuleValue), for the given MeasureID value
When I debug this, it seems to be running the trigger for EVERY record in the parent table, so I think I need to modify the trigger, but i"m not sure how to get the MeasureID of JUST the record that was added/deleted/modified.
I'm guessing it has something to do with the "magic tables" inserted, deleted, etc. but I can't seem to get the syntax right.
Thanks!

Answer 1

I would argue that unless you have a very good reason, storing values that can easily be computed on a query level is a mistake.
This seems like one of many cases I've seen where people think they gain something by storing values on one table that's calculated from values of another table, but in fact the opposite is true - now you have two points of data that needs to be synchronized at all times, and since the process synchronizing them is a trigger, you don't really have control over that - it's quite easy to disable / enable triggers, for instance.
Therefore, My advice to you would be to remove that trigger all together and simply calculate the value when you need to.
Please note that since SQL Server supports max() over(partition by) meaning you don't even need a group by if you want to calculate the max of a column.
Updated Following your comments to the answer, it seems like you have a good reason to store these values.
Having said all that, here's a direct answer to the question you've asked.
In SQL Server triggers, the database enables you to query two special tables called inserted and deleted. These tables contains the data that was (or going to be, in case of instead of triggers) inserted or deleted to the table on which the trigger is declared.
Please note that in SQL Server, triggers are fired per statement, not per row. This means that the inserted and deleted tables might contain 0, 1 or many rows.
If you still want to calculate the value using triggers, I would advise a trigger for insert/update and another trigger for deletes.
This would make for much simpler code.
In the delete trigger, you left join to the deleted table:
UPDATE T
SET QARTOD = MaxValue
FROM factMeasures As T
JOIN
(
SELECT d.fkMeasureID, Max(t.RuleValue) As MaxValue
FROM Deleted AS d
LEFT JOIN dt_QCflags As t
ON d.QC_ID = t.QC_ID
GROUP BY d.fkMeasureID
) as D
ON T.MeasureID = D.fkMeasureID
In the insert/update trigger, you write a very similar code - but you don't need to refer to the physical table in this case, only the inserted table:
UPDATE T
SET QARTOD = MaxValue
FROM factMeasures As T
JOIN
(
SELECT fkMeasureID, Max(RuleValue) As MaxValue
FROM Inserted
GROUP BY fkMeasureID
) as I
ON t.MeasureID = I.fkMeasureID