Tony Rogerson brings us an interesting blog post about T-SQL variable assignment and SET vs. SELECT. The issue? With SELECT you can assign values to multiple variables simultaneously. But with SET, you can set up your assignment such that you get an exception if more than one row is assigned to the variable. Both are desirable qualities… But unfortunately, as Tony shows us, it’s difficult to achieve both multiple assignment and getting the exception thrown, at the same time. Tony shows us a solution involving checking for the number of rows affected after the assignment. Creative and effective, but it still has an issue: Unlike with SET when it throws an exception, with Tony’s solution the variables will still have been affected by the assignment.
As I was reading Tony’s post, I couldn’t help but think that there must be another way. And low and behold, there is — at least, in SQL Server 2005. Thanks to the power of windowed aggregates we can have our multiple pieces of cake and eat them, all at the same time. Wonderful stuff.
So, here’s what you do: Set up a CTE that selects the columns you’d like to assign to your variables, and also get COUNT(*), partitioned by 1 (or some other arbitrary literal). By partitioning by a literal, we will end up with the row count for the entire set. In the outer query, express the assignments from the columns returned by the CTE, but add an additional WHERE clause that compares the value of the COUNT(*) column with a subquery against a table of numbers. In the following example which I’ve adapted from Tony’s blog, I’m using master..spt_values for the numbers, but you are encouraged to use a properly-indexed table of numbers, should you decide to use this technique:
DECLARE @reserved INT, @rowcnt INT, @used INT SET @reserved = -1 SET @rowcnt = -1 SET @used = -1 ;WITH x AS ( SELECT reserved, rowcnt, used, COUNT(*) OVER(PARTITION BY 1) AS theCount FROM sysobjects so INNER JOIN sysindexes si ON si.id = so.id WHERE so.name = ‘sysrowsets’ ) SELECT @reserved = reserved, @rowcnt = rowcnt, @used = used FROM x WHERE theCount = ( SELECT number FROM master..spt_values WHERE TYPE = ‘p’ AND number BETWEEN 1 AND theCount ) SELECT @reserved, @rowcnt, @used
As you’ll see if you run this on your end, an exception is thrown and the values of the variables are not affected. This works because the subquery used in the WHERE clause will return more than one value if theCount is greater than 1, thereby violating the rule that subqueries must only return one value.
The price you’ll pay for this convenience? Extremely complex code for a simple variable assignment, in addition to a slight performance penalty. Is it worth it? Probably not, at least for me. To be honest, I seriously doubt I will ever use this — I’ve never been especially concerned with the chance of multiple rows screwing up my variable assignment, and those times that it has happened, I’ve remedied the situation other ways (e.g., defining a better primary key). That said, I think this was an interesting T-SQL challenge, and if anyone comes up with a more elegant solution than Tony’s or mine, I’d love to see it!