Question

I have a weird requirement which I need to use inside my Stored Procedure in SQL Server 2008 R2.

I need a FIRST aggregate function which returns the first element of a sequence and I will use that with HAVING clause.

Let me give you an example:

DECLARE @fooTable AS TABLE(
    ID INT,
    CategoryName NVARCHAR(100),
    Name NVARCHAR(100),
    MinAllow INT,
    Price DECIMAL(18,2)
);

INSERT INTO @fooTable  VALUES(1, 'Cat1', 'Product1', 2, 112.2);
INSERT INTO @fooTable  VALUES(2, 'Cat2', 'Product2', 4, 12.34);
INSERT INTO @fooTable  VALUES(3, 'Cat1', 'Product3', 5, 233.32);
INSERT INTO @fooTable  VALUES(4, 'Cat3', 'Product4', 4, 12.43);
INSERT INTO @fooTable  VALUES(5, 'Cat3', 'Product5', 1, 13.00);

DECLARE @minAllowParam AS INT = 3;

SELECT ft.CategoryName, SUM(ft.Price) FROM @fooTable ft
GROUP BY ft.CategoryName;

As you see, we have a table and some dummy values. Inside the SELECT query, we group the categories together and sum the price of the products up.

This query returns the following result:

CategoryName     TotalPrice
---------------- ----------------
Cat1              345.52
Cat2              12.34
Cat3              25.43

What I need here is something like this:

SELECT ft.CategoryName, SUM(ft.Price) FROM @fooTable ft
GROUP BY ft.CategoryName
HAVING GetFIRST(MinAllow) >= @minAllowParam;

In that our case with a query something like this, we should be able to select following results:

INSERT INTO @fooTable  VALUES(2, 'Cat2', 'Product2', 4, 12.34);
INSERT INTO @fooTable  VALUES(4, 'Cat3', 'Product4', 4, 12.43);
INSERT INTO @fooTable  VALUES(5, 'Cat3', 'Product5', 1, 13.00);

As the INSERT INTO @fooTable VALUES(1, 'Cat1', 'Product1', 2, 112.2); record is the first element of a sequence and has the value of 2 for MinAllow column, Cat1 should be out of scope here. On the other hand, INSERT INTO @fooTable VALUES(5, 'Cat3', 'Product5', 1, 13.00); record has the value of 1 for MinAllow column but is the second element of the sequence. So, Cat3 is safe and can be selected.

Note: MIN or MAX is not what I am looking for!

I know that this example logically does not make sense but I have a situation which it totally does and hard to explain here on the other hand.

Any thoughts?

Edit:

I am assuming that I can achieve what I want here by creating a CLR
User-Defined Aggregate Function but I do not want to do this if there
is any other choice

Answer 1

How about

SELECT f1.CategoryName, SUM(f1.Price) 
FROM @fooTable AS f1
INNER JOIN (
    SELECT MinAllow, CategoryName
    FROM (
         SELECT MinAllow, CategoryName, ROW_NUMBER() OVER (PARTITION BY CategoryName ORDER BY ID) AS m
         FROM @fooTable
    ) AS f
    WHERE m = 1
) AS f2 ON f1.CategoryName = f2.CategoryName
WHERE f2.MinAllow >= @minAllowParam
GROUP BY f1.CategoryName

I know not a very elegant query. Maybe I can tweak it a little if I work on it a little longer!

Edit: Ok the inner most subquery should be unnecessary. This should also work:

SELECT f1.CategoryName, SUM(f1.Price) 
FROM @fooTable AS f1
INNER JOIN (
    SELECT MinAllow, CategoryName, ROW_NUMBER() OVER (PARTITION BY CategoryName ORDER BY ID) AS m
    FROM @fooTable
) AS f2 ON f1.CategoryName = f2.CategoryName
WHERE f2.m = 1 AND f2.MinAllow >= @minAllowParam
GROUP BY f1.CategoryName