Question

In: Computer Science

Consider the following database schema: LIKE(person, sport), PRACTICE(person, sport), where person and sport are keys in...

Consider the following database schema:

LIKE(person, sport),

PRACTICE(person, sport),

where person and sport are keys in both tables. The table LIKE gives the sports a person likes, the table PRACTICE gives the sports a person practices. We assume that a person likes at least one sport and practices at least one sport. We assume also that a person does not like a sport if the sport is not listed among the sports that person likes.

Express the following queries in relational algebra

  1. List the people who practice at list one sport they like
  2. List the people who practice at least one sport they do not like
  3. List pairs of people who practice at least one common sport
  4. List the people who like all the sports they practice
  5. List the people who practice all the sports they
  6. List the people who practice all the sports John

Solutions

Expert Solution

1. LIKE.person LIKE.person = PRACTICE.person ^ LIKE.sport = PRACTICE.sport (LIKE X PRACTICE)
2. person ( (PRACTICE) - (LIKE) )
3. a.person, b.person a.sport = b.sport ^ a.person != b.person (PRACTICE a X PRACTICE b)
// self join practice where sport is same but people different
4. person ( (PRACTICE) (LIKE) )
// whatever they practice is a subset of sports they Like
5. person ( (LIKE) (PRACTICE) )
// whatever they like is a subset of sports they Practice
6. person ( (PRACTICE) (( sport (LIKE) ) ( sport (PRACTICE)) ) )
// Practice relation is divided by all the sports in the union of Like and Practice

venereology answered 6 months ago
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