XQuery syntax to evaluate math formula

Question

I want to evaluate a math formula inside of a function, so I can't use dynamic SQL.
I found that the query on an XML-variable with a literal works well.
 DECLARE @sParsedFormula varchar(200);
 DECLARE @xFormula xml;
 SET @xFormula = '';
 SET @sParsedFormula = '1+2';
 SELECT @xFormula.query('3+3') , @xFormula.query('sql:variable("@sParsedFormula")')
Output (1st value is correctly evaluated, 2nd not):
"6", "1+2"
I can't directly pass @sParsedFormula to the query otherwise I get "The argument 1 of the XML data type method "query" must be a string literal".
What is the correct XQuery-syntax to evaluate the SQL:variable?
Tx,
Patrick

Answer 1

You are obviously on the wrong track when you ask for the impossible.
You can use sql:variable in XQuery to provide a value. You cannot provide an
expression.
If you want to pursue the set-up, your function will have to parse the expression and evalaute it. You can do this is in T-SQL, but it is not the best language for string parsing. Count on wast^H^H^H^Hspending quite a few hours on the development.
If you do it in C#, undoubtedly it will be easier, and I would expect that you can find classes that performs the parsing and evaluation. This can reduce your development costs considerably.
Of the languages I have worked with, there is one where the evaluting the expressions are dirt simple: Perl. Here you can just say
  $value = eval($expr)
But running Perl code from SQL Server? Nah.
If you want to do this in T-SQL only, it will have to be dynamic SQL. To avoid the cursor you can do somthing like:
   SELECT @sql =
     (SELECT 'SELECT ' + ltrim(str(id)) + ', ' + expr + ';' + char(13) + char(10)
      FROM   #temp
      FOR XML PATH(''), TYPE).value('.', 'nvarchar(MAX)')
  PRINT @sql
  INSERT #newtemp(id, value)
     EXEC (@sql)
  UPDATE #temp
  SET    value = n.value
  FROM   #temp t
  JOIN   #newtemp n ON t.id = n.id
So, yes, XML had a place in your solution. But where you were looking.
But note that with this solution, you will lose it all if one expression does not parse.
Erland Sommarskog, SQL Server MVP, [email protected]