The Lookup transformation is normally used to query the lookup source based on the lookup ports in the transformation and a lookup condition. The Lookup transformation returns the result of the lookup. In this app, the lookup is used to return the number of occurrences of key value in the lookup source by using lookup override. Business Case :Consider the input from a flat file which contains list of unique customer records. There is a relational table which contains list of orders for each customer. The target flat file has to be populated with the customer ID and order count.