SQL → Relational Algebra

Database Schema

Frequents(Kid, Store)
Sells(Store, Candy)
Likes(Kid, Candy)

설명

Frequents

→ 어떤 kid가 어떤 store에 자주 가는지

Sells

→ 어떤 store가 어떤 candy를 파는지

Likes

→ 어떤 kid가 어떤 candy를 좋아하는지

모든 attribute는 string이다.

Question 1

List the stores that sell at least one kind of candy that each kid likes as kid-store pairs.

If there is no such store for a kid, show null paired with that kid.

SQL

SELECT K.Kid, S.Store
FROM Likes K
LEFT JOIN Sells S
ON K.Candy = S.Candy;

Relational Algebra

Step1

R1 ← Likes ⨝Candy Sells