Question

I want to create conditional join to a table in my T-SQL query. Tables used in this example are from Northwind database (with only one additional table ProductCategories)

Table Products and table Categories have many-to-many relationship, hence table ProductCategories comes into picture.

I need the sum of Quantity column on table OrderDetails for each of the products falling under certain category. So I have a query like one below

Select p.ProductName, Sum(od.Quantity) As Qty
From Products p
    Join OrderDetails od On od.ProductID = p.ProductID
    Join ProductCategories pc On pc.ProductID = p.ProductID
        And pc.CategoryID = @CategoryID
Group By p.ProductName

@CategoryID is an optional parameter. So in case it’s not supplied, the join to table ProductCategories will not be required
and query should look like one below

Select p.ProductName, Sum(od.Quantity) As Qty
From Products p
    Join OrderDetails od On od.ProductID = p.ProductID
Group By p.ProductName

I want to achieve this without repeating the whole query with If conditions (as below)

If @CategoryID Is Null
    Select p.ProductName, Sum(od.Quantity) As Qty
    From Products p
        Join OrderDetails od On od.ProductID = p.ProductID
    Group By p.ProductName
Else
    Select p.ProductName, Sum(od.Quantity) As Qty
    From Products p
        Join OrderDetails od On od.ProductID = p.ProductID
        Join ProductCategories pc On pc.ProductID = p.ProductID
            And pc.CategoryID = @CategoryID
    Group By p.ProductName

This is simplified version of the query which has many other tables and conditions like ProductCategories. And will require multiple multiple If conditions and repetition of the query. I have also tried dynamically generating the query. It works but query is not readable at all.

Any solutions?
Thank you.

Answer 1

Try this, if you’ll use properly parametrized query – there will be no performance impact but may be gain:

Select p.ProductName, Sum(od.Quantity) As Qty
From Products p
    Join OrderDetails od On od.ProductID = p.ProductID
WHERE @CategoryID IS NULL OR EXISTS (SELECT * FROM ProductCategories WHERE CategoryID = @CategoryID AND ProductID = p.ProductID)
Group By p.ProductName

Actually

in your query if there can be multiple rows in ProductCategories for one row in OrderDetails – then you get duplicates of od.Quantity in your SUM – is it an intended behavior?