what is the main difference between lookup ,join and merge in DataStage

LOOK UP STAGE:
* One output link and one reject link.

* Faster if the volume of data is less than the memory size.

* If the size is too huge then DS uses scrap files to store the
   remaining data and that hinders the performance.



JOIN STAGE:
* Can have any no. of input links

* Does not use memory, look up is done on table level,
   so would be better to use if the data volume is pretty  huge.

* Doesn't have any reject link.

* Only one output link.



MERGE STAGE:
*Can have any number of input link and update

  links.

* Need to have a key column.

* Can have any number of reject link.

* Only one output link.