what are conditional statements in aggregator transformation?

The conditional statements are the statements that we can include in the aggregate function itself.

While performing aggregate calculations.

EX:-

Sum (IIF(dept=’HR’,salary,0))   

here if dept is HR only will do SUM.

When cache files deleted for incremental aggregation what happens? 

When cache files not available in informatica server, then session won't fail, it will recreate the cache automatically. But it takes more time compared to normal run, reason is recreation of cache takes some time.

Can we select both sorted input and incremental aggregation at a time? 
No, we can't select. Reason is if you select sorted input it performs calculations in memory. When you select incremental aggregation it performs calculations on existing cache values.

Thanks 

Hari Babu