Creating SQL UPDATE statements on the fly

Question

I am in the midst of updating data in multiple tables. Currently I have a table that has one field, "sources", that is just a list of all tables that include the field "itemid". I also have a table that has 2 fields, "itemid" and "olditemid". In TSQL, I would like to iterate through the sources and create the update statements on the fly. Here is what I was trying to do but I get some errors in the update statement that my variable is not declared. I am not sure this is even close the correct way I should be doing this. Ideas?

DECLARE @tblName varchar(50)

DECLARE process_cursor CURSOR FOR 
    SELECT source 
    FROM tmpTableNames

OPEN process_cursor

FETCH NEXT FROM processcursor 
INTO @tblName

WHILE @@FETCH_STATUS = 0

    UPDATE @tblName 
        SET itemid = r.itemid 
        FROM @tblName v, itemref r
        WHERE r.olditemid = v.itemid

    FETCH NEXT FROM process_cursor
    INTO @tblName

END
CLOSE processcursor
DEALLOCATE processcursor

Answer 1

What you are trying to do is referred to as "dynamic SQL". While you're on the right track, you can't simply stick a variable in place of an object name and execute the query. I'll leave the pitfalls of dynamic SQL to someone else. What you're looking for is this:

DECLARE @tblName varchar(50)

DECLARE process_cursor CURSOR FOR 
    SELECT source 
    FROM tmpTableNames

OPEN process_cursor

FETCH NEXT FROM processcursor 
INTO @tblName

WHILE @@FETCH_STATUS = 0
BEGIN
    DECLARE @sql NVARCHAR(500)

    SELECT @sql = 'UPDATE [' + @tbleName + '] SET itemid = r.itemid FROM [' + @tbleName + '] v, itemref r WHERE r.ilditemid = v.itemid'

    EXEC sp_executesql @sql
    FETCH NEXT FROM process_cursor
    INTO @tblName

END
CLOSE processcursor
DEALLOCATE processcursor

What this does is turn your update query into a string, then passes the SQL contained in that string to the sp_executesql stored procedure (this is the recommended way of executing dynamic sql, rather than EXEC('foo')).