Week 10 Lecture (36)

36

Mapping SQL to Relational Algebra

A given SQL query typically has many translations to RA.

For example:

SELECT s.name, e.subj
FROM   Students s, Enrolments e
WHERE  s.id = e.sid AND e.mark > 50;

is equivalent to any of

π_{s.name,e.subj}( σ_{s.id=e.sid ∧ e.mark>50} ( Students × Enrolments ) )
π_{s.name,e.subj}( σ_s.id=e.sid ( σ_e.mark>50 ( Students × Enrolments ) ) )
π_{s.name,e.subj}( σ_e.mark>50 ( Students ⋈_s.id=e.sid Enrolments ) ) )