In order to avoid nested select statements we can go 
for "FOR ALL ENTRIES".First fetch values from database 
table to internal table and only for the values in the 
internal table we can fetch values from other database 
table based on conditions in the where clause.

it is mandetory that we have to declare allthe primary keys 
in either select Stmt or in where condition. orelse 
duplicate records will not get selected.

to avoid duplaction of records

For ALL Entries is used to join two different tables. By
tables i mean the internal tables. For All Entries has an
internal where condition in it. which makes the selection
process much faster. But still it is always better to use
inner joins when you have to join two database tables.