How to retrieve only a part of entire dataset (similar to LIMIT in MySQL)?

To retrieve first n rows in a query, use the FIRST clause:

select FIRST 10 *
from employee
order by last_name;

To get the last n records, invert the ORDER BY clause:

select FIRST 10 *
from employee
order by last_name DESC;

Please note that using the FIRST clause without ORDER BY doesn't make much sense as relational database systems don't have a concept of row number. Unless you want 'any 10 records', you should not rely on the order of records if you don't use ORDER BY, as it may change after backup and restore.

If you wish to get a set of records that is not at the start of dataset, for example, you wish to get N records from record X to record X+N, use the skip clause to skip the first X records:

select FIRST 10 SKIP 20 *
from employee
order by last_name;

Also, please note that NULLs always get at the end of dataset, as they are unknown values. If you wish, you can use NULLS FIRST clause to have them at the beginning when doing the descending sort (or whatever combination you prefer).

Firebird 2 introduces a similar ROWS clause that is prefered as it is compliant with SQL standard, can be used with UNION and any type of subquery.

select *
from employee
order by last_name
rows 20 to 30;