Question

How do I make the following T-SQL statement legal? I can copy the subquery that sets @Type variable for every CASE option, but I’d rather execute the subquery only once. Is it possible?

SELECT
      @Type = (SELECT CustomerType FROM dbo.Customers WHERE CustomerId = (SELECT CustomerId FROM dbo.CustomerCategories WHERE CatId= @CatId)),
      CASE
         WHEN @Type = 'Consumer'THEN dbo.Products.FriendlyName
         WHEN @Type = 'Company' THEN dbo.Products.BusinessName
         WHEN @Type IS NULL THEN dbo.Products.FriendlyName
         WHEN @Type = '' THEN dbo.Products.FriendlyName
      END Product,
      ...
FROM
      Products
INNER JOIN
      Category
...

Edit: modified my example to be more concrete…have to run now…will be back tomorrow…sorry for signing off short but have to pick up kids 😀 will check back tomorrow. THX!!

Clarification: I can’t separate the two: in the subquery’s where-clasue, I need to refer to columns from tables that’re used in the main query’s join stmt. If I separate them, then @Type will lose relevance.

Answer 1

Why not just separate it into two operations? What do you think you gain by trying to glom them into a single statement?

SELECT @Type = (subquery);

SELECT CASE WHEN @type = 'Consumer'...

At the risk of sounding obtuse, do you really need the variable at all? Why not:

SELECT CASE WHEN col_form_subquery = 'Consumer' THEN ...
  END Product
FROM (<subquery>) AS x;

With that form you’ll need to decide whether you want to assign values to variables or retrieve results.

You can also pull multiple variables, e.g.

SELECT @Col1 = Col1, @Col2 = Col2
  FROM (<subquery>) AS x;

-- then refer to those variables in your other query:

SELECT *, @Col1, @Col2 FROM dbo.Products WHERE Col1 = @Col2;

But this is all conjecture, because you haven’t shared enough specifics.

EDIT okay, now that we have a real query and can understand a bit better what you’re after, let’s see if we can write you a new version. I’ll assume that you were only trying to store the @Type variable so you can re-use it within the query, and that you weren’t trying to store a value there to use later (after this query).

SELECT CASE 
      WHEN c.CustomerType = 'Company' THEN p.BusinessName
      WHEN COALESCE(c.CustomerType, '') IN ('Consumer', '') THEN p.FriendlyName
    END
    --, other columns
FROM dbo.Products AS p
INNER JOIN dbo.Category AS cat 
  ON p.CatId = cat.CatId
INNER JOIN dbo.CustomerCategories AS ccat
  ON ccat.CatId = cat.CatId
INNER JOIN dbo.Customers AS c
  ON c.CustomerId = ccat.CustomerId
WHERE cat.CategoryId = @CatId;

Some notes:

• I’m not sure why you thought subqueries are the right way to approach this. Usually it is much better (and clearer to other developers) to build proper joins and let SQL Server optimize the query overall instead of trying to be smart and optimize individual subqueries largely independent of the main query. A proper join will help to eliminate rows up front that would otherwise, through the subqueries, potentially be materialized – only to be discarded. Trust SQL Server to do its job, and in this case its job is to perform a join across multiple tables.
• The join to dbo.Category might not be needed if the SELECT doesn’t need to display the category name. If so then change the where clause and remove that join (join to CusomterCategories instead).
• The second case can be changed to a simple ELSE if you’ve covered all the possible scenarios.
• I made an assumption about the join between Products and Category (why is Category not plural like the others?). If this isn’t it please fill us in.