Show the names of employees with a certificate.
select t1.name from employee as t1 join certificate as t2 on t1.eid = t2.eid
Which of them have a certificate on Boeing 737-800?
select t1.name from employee as t1 join certificate as t2 on t1.eid = t2.eid join aircraft as t3 on t3.aid = t2.aid where t3.name = "Boeing 737-800"
Show the names for all employees without this particular certificate.
select name from employee except select t1.name from employee as t1 join certificate as t2 on t1.eid = t2.eid join aircraft as t3 on t3.aid = t2.aid where t3.name = "Boeing 737-800"