[michael1999]

How would that work? Your schema allows an employee to be assigned to multiple departments, and the query must count them.

[atwebb]

It is also called Table Elimination if you want a google-able term.

[jmalicki]

Because of the foreign key, the query should reduce to "explain select count(*) from Department d;"

[doctor_eval]

The schema only allows one employee per department, so there is no need to look up the employee table.