There really isn't enough information to correctly answer your question. Dbeaver is just an application that allows you to execute SQL against your database, you could by using mysql, sql server, postgres...etc.
But we can make some assumptions. What you are asking is "how to join 2 tables in sql". Given the commonality of Department + Employee tables I think we can assume that the Employee Table has a foreign key (so a relationship) on the Employee Table that points to the Department Tables Primary key (the identifying column of that table).
So for example, Maybe on Employee Table you have DepartmentId as the foreign key which points to DepartmentId on the Department table.
You're query would look like this then
SELECT
A.Department_name, B.first_name, B.last_name
FROM Department AS A
JOIN Employee AS B ON B.DepartmentId = A.DepartmentId
From there you can add where/order by clauses as you see fit.
