Basic Query Structure

Queries in SQL have the following query structure:

select A1, A2, ..., An
from r1, r2, ..., rm
where P

This is equivalent to the following relational algebra expression, and will output a relation.

$\Pi _{A_1, A_2, ..., A_n} (\sigma _P (r_1 \times r_2 \times ...\times \, r_m))$