Lookup is a transformation
It look up the values from a relational table/view or a flat file. The developer defines the lookup match criteria. There are two types of Lookups in Powercenter-Designer, namely; 1) Connected Lookup 2) Unconnected Lookup . Different caches can also be used with lookup like static, dynamic, persistent, and shared(The dynamic cache cannot be used while creating an un-connected lookup). Each of these has its own identification. For more details, the book "Informatica Help" can be useful.
Hope you are aware with the basics of Informatica. Now proceeding through lookup transformation.
Lookup transformation is Passive and it can be both Connected and UnConnected as well. It is used to look up data in a relational table, view, or synonym. Lookup definition can be imported either from source or from target tables.
For example, if we want to retrieve all the sales of a product with an ID 10 and assume that the sales data resides in another table called 'Sales'. Here instead of using the sales table as one more source, use Lookup transformation to lookup the data for the product, with ID 10 in sales table.
Difference between Connected and UnConnected Lookup Transformation:
1. Connected lookup receives input values directly from mapping pipeline whereas UnConnected lookup receives values from: LKP expression from another transformation.
2. Connected lookup returns multiple columns from the same row whereas UnConnected lookup has one return port and returns one column from each row.
3. Connected lookup supports user-defined default values whereas UnConnected lookup does not support user defined values.
