Question

So I got a problem looks like with string conversion. I have tried everything under the sun. My goal is to inert the values from the loop into the table. However I can get the field value but I can not get the table name value due to sql thinks I am calling a table rather than a string. I have tried casting converting everything. Here is the code sample:

DECLARE @C varchar(65)
DECLARE @FieldName varchar(50)
SET @FieldName = 'ProjectID'
SELECT @C = MIN(TABLE_NAME) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE = 'Base Table'

CREATE TABLE #temp_t (
    [field] varchar(100),
    [tblname] varchar(100)
    )

WHILE @C is not null
    BEGIN
        DECLARE @qbExists int
        DECLARE @tblname varchar(100)
        SET @tblname = CONVERT(varchar(100), @C)
        SELECT @qbExists = COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = @C AND COLUMN_NAME = @FieldName
        IF @qbExists = 1 AND 
           @C <> 'TimeExpenseUnionQuery' AND 
           @C <> 'TimeExpense' AND
           @C <> 'BC_BA' AND
           @C <> 'BC_BE' AND
           @C <> 'BC_EL' AND
           @C <> 'BC_TE'
          BEGIN
            EXEC('
                  SELECT ' + @FieldName + ', ' + @C + ' FROM ' + @tblName +'
                 ')
          END

        SELECT @C = MIN( TABLE_NAME ) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME > @C
END

SELECT * FROM #temp_t 

DROP TABLE #temp_t

The problem child lies within the SELECT ‘ + @FieldName + ‘, ‘ + @C + ‘ FROM ‘ + @tblName.
Now if I do not use the @C it works fine. If I try to CONVERT or CAST the @C I get an invalid column name? What gives?

Answer 1

You need to quote it to make it a constant, which requires doubling up because of dynamic SQL

EXEC('
              SELECT ' + @FieldName + ', ''' + @C + ''' FROM ' + @tblName +'
             ')

Whether the use of dynamic SQL is a good idea or not is a different matter…