finding records in one table but not the other

[oracle_faq]

Another approach would be using minus

select col1 alias1, col2 alias2
from table1
where something = nothing
minus
select col3 alias1, col2 alias2
form table2
where something = everything

This may be faster in certain cases because you are not doing a lookup in second table for every record in the first table especially when your where clause may have unindexed criteria only.