In this tutorial I will explain and give a simple example of SSIS lookup transformation.
Let's start with short explanation of the term lookup. Lookup takes input value and searches for a row in table that contains this value in the specified field; once it finds it you are able to extract values from different fields that belong to the same "row".
Below is a simple example. I have a country table which contains field ID and country (right side). My input value is country with value UK and I want to get ID for this country.
Lookup takes UK then searches Country Field for the entry and when it finds it it returns the ID which 3.
Now that we covered basics let's explain when you would use lookup. There are multiple scenarios however I will limit myself to explain one. Because you use SSIS you most likely are involved in building a data warehouse. During your extract you get source system key (business key) that you want to find in your dimension table and replace with ID (surrogate key) which will be used to load your Fact Table.
NOTE: This example is a simple lookup example using dimension table that does not track history so it should be used only for training purposes and specific scenarios. I will try to write more articles covering typical data warehouse lookups soon.
In this section I will show you how to configure SSIS Lookup transformation (if you are new to SSIS check How to create SSIS package)
Create a new package add data flow in source control. Go to data flow and add source item; I will use OLE DB Source extracts Country field and I will use this field as input for lookup. See below screenshot of data flow and sample of data.