Renamerenameρ x (E): the result of E, named x. ρ x(A1, …, An) (E) also renames the attributes. changes names. It does not change tuples.

ρ teacher (instructor)                                the relation is now called teacher
ρ staff(staff_id, staff_name, dept, pay) (instructor) also renames the 4 attributes, in order

In SQL, both are AS:

SELECT id AS staff_id, name AS staff_name, dept_name AS dept, salary AS pay
FROM instructor AS staff;

Why rename

• The result of an expression has no name. ρ gives it one, so a later operator can refer to its attributes as staff.pay.
• An expression that uses the same relation twice needs two different names for it, such as ρ a (department) and ρ b (department). The next lesson uses this.