Why use the lookup transformation ?

Lookup table is nothing but the lookup on a table,view,synonym and falt file.by using lookup we can get a related value with join conditon and performs caluclations

two types of lookups are there

1)connected  2)Unconnected

connected lookup is with in pipeline only but unconnected lookup is not connected to pipeline

unconneceted lookup returns single column value only

What r the types of lookup?

Connected, unconnected, cache, uncached

Differences between connected and unconnected lookup?

Connected lookup

Receives input values diectly from the pipe line.

U can use a dynamic or static cache

Cache includes all lookup columns used in the maping

Support user defined default values

Unconnected lookup

Receives input values from the result of a  lkp expression in a another transformation.

U can use a static cache.

Cache includes all lookup out put ports in the lookup condition and the lookup/return port.

Does not support user defiend default value

what is meant by lookup caches?

lookup cache is the temporary memory that is created by the informatica server to hold the lookup data and to perform the lookup conditions