Friend Suggestions by Mutual Friends

SQL practice question · Hard · Joins · Meta

friendships is undirected: a pair may be stored as (a, b), as (b, a), or both; rows where user_a = user_b are bad data and must be ignored. Suggest pairs of users who are NOT friends but have at least 2 mutual friends. Return user_1 (smaller id), user_2 (larger id) and mutual_friends. Order by mutual_friends DESC, user_1, user_2.

Table: friendships

Schema of friendships
Column	Type
user_a	INTEGER
user_b	INTEGER

Schema of friendships

Column

Type

user_a

INTEGER

user_b

INTEGER

Sample rows of friendships
user_a	user_b
1	2
1	3
2	4
3	4
4	5
1	5
2	3

Sample rows of friendships

user_a

user_b

Expected output on the sample data
user_1	user_2	mutual_friends
1	4	3
2	5	2
3	5	2

Expected output on the sample data

user_1

user_2

mutual_friends

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