The Integration Service queries the lookup based on the ports and properties you configure in the Lookup transformation. The Integration Service runs a default lookup query when the first row enters the Lookup transformation.
If you use a lookup against a relational table, you can override the lookup query.
You can use the override to add a WHERE clause or transform the lookup data before it is cached.
If you configure a SQL override and a filter on the lookup query, the Integration Service ignores the filter.