If you need to override the default join, you can enter contents of the WHERE clause that specifies the join in the custom query. If the query performs an outer join, the Integration Service may insert the join syntax in the WHERE clause or the FROM clause, depending on the database syntax.
You might need to override the default join under the following circumstances:
Columns do not have a primary key-foreign key relationship.
The datatypes of columns used for the join do not match.
You want to specify a different type of join, such as an outer join.