Executing dynamic SQL in a SQLServer 2005 function

Question

I will preface this question by saying, I do not think it is solvable. I also have a workaround, I can create a stored procedure with an OUTPUT to accomplish this, it is just easier to code the sections where I need this checksum using a function.

This code will not work because of the Exec SP_ExecuteSQL @SQL calls. Anyone know how to execute dynamic SQL in a function? (and once again, I do not think it is possible. If it is though, I'd love to know how to get around it!)

Create Function Get_Checksum
(
    @DatabaseName      varchar(100),
    @TableName         varchar(100)
)
RETURNS FLOAT
AS
BEGIN

 Declare @SQL        nvarchar(4000)
 Declare @ColumnName varchar(100)
 Declare @i          int
 Declare @Checksum   float
 Declare @intColumns table (idRecord int identity(1,1), ColumnName varchar(255))
 Declare @CS         table (MyCheckSum bigint)

 Set @SQL = 
        'Insert Into @IntColumns(ColumnName)' + Char(13) + 
        'Select Column_Name' + Char(13) +
        'From   ' + @DatabaseName + '.Information_Schema.Columns (NOLOCK)' + Char(13) +
        'Where  Table_Name = ''' + @TableName + '''' + Char(13) +
        '       and Data_Type = ''int''' 

 -- print @SQL

 exec sp_executeSql @SQL

 Set @SQL = 
        'Insert Into @CS(MyChecksum)' + Char(13) + 
        'Select '

 Set @i = 1

 While Exists(
       Select 1
       From   @IntColumns
       Where  IdRecord = @i)
 begin
       Select @ColumnName = ColumnName
       From   @IntColumns
       Where  IdRecord = @i

       Set @SQL = @SQL + Char(13) + 
            CASE WHEN @i = 1 THEN 
                 '    Sum(Cast(IsNull(' + @ColumnName + ',0) as bigint))'
                 ELSE
                 '    + Sum(Cast(IsNull(' + @ColumnName + ',0) as bigint))'
            END

       Set @i = @i + 1
 end

 Set @SQL = @SQL + Char(13) + 
      'From ' + @DatabaseName + '..' + @TableName + ' (NOLOCK)'

 -- print @SQL

 exec sp_executeSql @SQL

 Set @Checksum = (Select Top 1 MyChecksum From @CS)

 Return isnull(@Checksum,0)

END
GO

Answer 1

It "ordinarily" can't be done as SQL Server treats functions as deterministic, which means that for a given set of inputs, it should always return the same outputs. A stored procedure or dynamic sql can be non-deterministic because it can change external state, such as a table, which is relied on.

Given that in SQL server functions are always deterministic, it would be a bad idea from a future maintenance perspective to attempt to circumvent this as it could cause fairly major confusion for anyone who has to support the code in future.

Answer 2

Here is the solution

Solution 1: Return the dynamic string from Function then

Declare @SQLStr varchar(max) 
DECLARE @tmptable table (<columns>)
set @SQLStr=dbo.function(<parameters>)
insert into @tmptable
Exec (@SQLStr)

select * from @tmptable

Solution 2: call nested functions by passing parameters.

Answer 3

You can get around this by calling an extended stored procedure, with all the attendant hassle and security problems.

http://decipherinfosys.wordpress.com/2008/07/16/udf-limitations-in-sql-server/

http://decipherinfosys.wordpress.com/2007/02/27/using-getdate-in-a-udf/

Answer 4

Because functions have to play nicely with the query optimiser there are quite a few restrictions on them. This link refers to an article that discusses the limitations of UDF's in depth.

Answer 5

Thank you all for the replies.

Ron: FYI, Using that will throw an error.

I agree that not doing what I originally intended is the best solution, I decided to go a different route. My two choices were to use sum(cast(BINARY_CHECKSUM(*) as float)) or an output parameter in a stored procedure. After unit testing speed of each, I decided to go with sum(cast(BINARY_CHECKSUM(*) as float)) to get a comparable checksum value for each table's data.