This is sourced from the Stanford Coursera self study DB class SQL quizzes.
Students at your hometown high school have decided to organize their social network using databases. So far, they have collected information about sixteen students in four grades, 9-12. Here's the schema:
Highschooler (ID, name, grade)
English: There is a high school student with unique ID and a given first name in a certain grade.
Friend (ID1, ID2)
English: The student with ID1 is friends with the student with ID2. Friendship is mutual, so if
(123, 456)is in the Friend table, so is
Likes (ID1, ID2)
English: The student with ID1 likes the student with ID2. Liking someone is not necessarily mutual, so if
(123, 456)is in the Likes table, there is no guarantee that
(456, 123)is also present.
DB is here
Prompt: Find the number of students who are either friends with Cassandra or are friends of friends of Cassandra. Do not count Cassandra, even though technically she is a friend of a friend.
My answer (which works) is below, but I am wondering if there is a more succinct way of accomplishing the same results. Any feedback would be appreciated.
select count(*)-1 from ( select id2 from friend f, highschooler a where a.name='Cassandra' and a.id=f.id1 union select id2 from friend f, highschooler a where a.id=f.id1 and f.id1 in ( select id2 from friend f, highschooler a where a.name='Cassandra' and a.id=f.id1 ) )