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T-SQL Variables: Multiple Value Assignment

6

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!

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Adam Machanic helps companies get the most out of their SQL Server databases. He creates solid architectural foundations for high performance databases and is author of the award-winning SQL Server monitoring stored procedure, sp_WhoIsActive. Adam has contributed to numerous books on SQL Server development. A long-time Microsoft MVP for SQL Server, he speaks and trains at IT conferences across North America and Europe.

6 COMMENTS

  1. Hi Adam,
    A similar effect can be obtained (with a more efficient plan and fewer keystrokes!) by using the ROW_NUMBER function in an ORDER BY clause on the assignment, as shown in this example:
    — Table representing some arbitrary result set
    DECLARE @T TABLE (id INT NOT NULL)
    — Simulate a query returning more than 1 row
    INSERT @T (id)
    SELECT 1 UNION ALL
    SELECT 2 UNION ALL
    SELECT 3;
    — The variable we will assign to
    DECLARE @id INTEGER;
    — Default value
    SET @id = -1;
    — The assignment statement
    — The ORDER BY clause will throw an exception
    — if row number 2 exists…
    SELECT @id = id
    FROM @T
    ORDER BY (1 / (ROW_NUMBER() OVER (ORDER BY (SELECT 1)) – 2));
    — Untouched value if more than one row was returned
    SELECT @id;

    The "ORDER BY (1 / (ROW_NUMBER() OVER (ORDER BY (SELECT 1)) – 2))" clause can be appended to any assignment statement (the ‘select 1’ part removes any dependency on the preceding query).
    If not saying for a moment that I’ll be using this routinely – but thanks for the puzzle!
    </Paul>

  2. what about trick with tinyint (MS SQL 2008)?
    DECLARE
       @reserved INT =-1,
       @rowcnt INT=-1,
       @used INT=-1,
       @control TINYINT=0
    SELECT
       @control=255*COUNT(*) OVER(PARTITION BY 1) ,
       @reserved =  reserved,
       @rowcnt =  rowcnt,
       @used = used
      FROM sysobjects so
       INNER JOIN sysindexes si ON si.id = so.id
       WHERE
           so.name = ‘sysrowsets’
    SELECT @reserved, @rowcnt, @used, @control

  3. This tweaks what has been said above?
    DECLARE
       @reserved INT,
       @rowcnt INT,
       @used INT
    SELECT
      @reserved =  reserved,
      @rowcnt =  rowcnt,
      @used = used
     FROM sysobjects so
      INNER JOIN sysindexes si ON si.id = so.id
      WHERE
          so.name = ‘sysrowsets’
     order by  case when COUNT(*) OVER(PARTITION BY 1) > 1 then cast(cast(COUNT(*) OVER(PARTITION BY 1) as varchar ) + ‘ Rows returned, expected only one.’ as int) end    
    — Should return:
    Msg 245, Level 16, State 1, Line 8
    Conversion failed when converting the varchar value ‘3 Rows returned, expected only one.’ to data type int.

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