Question

I’m working with SQL 2005, and have the following code:

DECLARE @exist4 INT
IF ( SELECT COUNT(*) existe
     FROM   sysobjects obj
     WHERE  obj.name = 'table4'
   ) > 0 
    BEGIN 
        SET @exist4 = 1
    END
ELSE 
    BEGIN 
        SET @exist4 = 0
    END

/*now if table4 exist i need add to my query but if i do something as it
i got next error
Invalid object name 'table4'.*/


IF ( @exist4 = 0 ) 
    BEGIN

        SELECT  [table].col1 ,
                [table].col2 ,
                [table].col3 ,
                [table].colN
        FROM    [table] ,
                table2 ,
                table3
        WHERE   [table].id = table1.id
                AND table3.id = table2.id
    END
ELSE 
    BEGIN
        SELECT  [table].col1 ,
                [table].col2 ,
                [table].col3 ,
                [table].colN
        FROM    [table] ,
                table2 ,
                table3 ,
                table4
        WHERE   [table].id = table2.id
                AND table3.id = table2.id
                AND table4.id = table3.id
    END

This code returns an error when Table4 does not exist. How can I get this query to work?

Answer 1

What is happening is that table4 does not exist at the time the query is parsed. That means you can’t run the query.

What you need to do is to delay the parsing of that part of the query until you know that table4 exists.

You can do this by putting the query in a string then running the stored procedure sp_excutesql. Something like this:

 DECLARE @myQueryString NVARCHAR(4000);
 SET @myQueryString = N'select table.col1, table.col2, table.col3, table.colN into #tmp
     from table,table2,table3,table4
     where table.id=table2.table1id 
     and table3.table2id=table2.id
     and table4.table3id=table3id';
 EXEC sp_executesql @myQueryString;

The part of the query that references table4 is only available as a string when the whole query is parsed the first time, so the parser sees just a string, not the reference to the various tables. When you run sp_executesql the string is then parsed and run. And since you put this inside your IF block it will only parse and run when it knows that table4 exists.