Purpose would be to achieve a SQL statement that would otherwise be impossible to achieve using a Source qualifier.
Source qualifier will only have the effect of a simple:
Select .... from ..... where .....
Now imagine wanting to merge two datasets from seperate tables that contain identical fields. You could do this by using 2 mappings, running two sessions in a batch.
(First A mappings and then B mappings)
The alternative is using ONE mapping with an SQL override that contains a union:
Select .... from table A
Union
Select .... from table B
There are various example you can think up that will let you handle some demand with SQL override, that would otherwise need more complex or diverse mappings
T. Blom
Information analyst
tbl@shimano-eu.com