Queries

[mmcclaf]

I have to make  some queries for 4 tables I have. The following
relations are:

Classes(class, type, country, numGuns, bore, displacement)
Ships (name, class, launched)
Battles (name, date)
Outcomes (ship, battle, result)

The three queries I'm stuck on are the following:

1. Find the classes that have only one ship as a member of that class
(not all ships are listed in the Ship table)
2. Find the countries that had both battleships and battlecruisers
(those fall under type in Classes)
3. Find those ships that "lived to fight another day"; they were
damaged in one battle, but later fought in another.

The best way for me to understand would be relational algebra for each
of the statements.

Any help in this would be greatly appreciated.