Lookup transformation in a mapping is used to look up data in a flat file, relational table, view, or synonym. We can also create a lookup definition from a source qualifier.
We have the following types of Lookup.
- Relational or flat file lookup. To perform a lookup on a flat file or a relational table.
- Pipeline lookup. To perform a lookup on application sources such as JMS or MSMQ.
- Connected or unconnected lookup.
- A connected Lookup transformation receives source data, performs a lookup, and returns data to the pipeline.
- An unconnected Lookup transformation is not connected to a source or target. A transformation in the pipeline calls the Lookup transformation with a: LKP expression. The unconnected Lookup transformation returns one column to the calling transformation.
- Cached or un-cached lookup.We can configure the lookup transformation to Cache the lookup data or directly query the lookup source every time the lookup is invoked. If the Lookup source is Flat file, the lookup is always cached.