Creating SQL UPDATE statements on the fly - sql

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')).

Answer 2

I don't think you can do it using a variable like that. You could use dynamic SQL for the update:
DECLARE #sql VARCHAR(1000)
SET #sql = 'UPDATE' + #tableName + etc..
EXEC ( #sql )
And just do this inside your cursor.

Answer 3

You can't execute sql dynamically like this - you need to pass a dynamically generated string into the exec function like 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
Declare #sql varchar(5000)
Select #sql = 'UPDATE ' + #tblName +
'SET itemid = r.itemid
FROM ' + #tblName + ' v, itemref r
WHERE r.olditemid = v.itemid'
Exec #sql
FETCH NEXT FROM process_cursor
INTO #tblName
END
CLOSE processcursor
DEALLOCATE processcursor

Answer 4

did you try
DECLARE #tblName varchar(50)?
I would think that would do it.

Answer 5

I've never been successful with variable-based UPDATE statements (i.e., UPDATE #tblName), unless I captured them into a string and executed these dynamically, as in:
EXEC 'UPDATE ' + #tblName + '
SET ItemId = (SELECT r.ItemId FROM itemref r WHERE r.OldItemId = ' + #tblName + '.itemId)'
For table TheTable, this should expand to:
EXEC 'UPDATE TheTable
SET ItemId = (SELECT r.ItemId FROM itemref r WHERE r.OldItemId = TheTable.ItemId)'