[use fiitpdt/postgres for this excercise] Suppose the following relational schema exists, where there are travel agents
selling flights to some destination. A flight can have multiple services from the set
{business class, lunch, drinks, priority boarding, online check-in}
.
AGENT (**a_id**, name)
FLIGHT (**destination**, pilot_name, a_id)
SERVICE (**flight**, **service**)