SQL & PL/SQL

Hello all
I'm a student on information systems and i have come to a stop...

i am trying to create a database and at one point i want to make a "check constraint" between 2 tables:
table A has a Start_Date and a End_Date columns
table B has an Unavailable_date column.

What i want to achieve is as Unavailable_date is populated with values,
when i try to insert values to table A it will check the existence of all the values in the Unavailable_date column in the range defined between start_date and end_date of that particular row entry.

I've read that check constraints cannot reference other tables and i should use triggers instead. No luck there, as some error happens at compilation and the trigger doesn't fire at all. Any ideas?

Btw can someone point me at a site/forum/something for oracle tutorials and info? I'm interested mostly on design of a db concerning problems like i explain above.

Edit: I'm working with SQL Developer

Message was edited by:
Jaguar