Write queries for follows and bookings

You are given tables/logs from a consumer app. Solve the following independent tasks.

Part A — Active following as of a date (SQL)

You have a follow event log:

• follow_events(follower_id, followee_id, event_type, event_ts)
• event_type ∈ {'follow','unfollow'}
• A user can follow/unfollow the same person multiple times.

Task: Given a parameter date D (inclusive), return all active follow relationships at the end of day D (i.e., after all events with event_ts <= D 23:59:59).

Output columns:

• as_of_date (value = D ), follower_id , followee_id

Clarifications/edge cases to handle:

• If the latest event on or before D is follow , the relationship is active.
• If the latest event is unfollow , it is inactive.
• If there are no events on/before D , it should not appear.

Part B — Add reciprocal friendships (SQL)

You have a directed friendship table:

• friendships(user_id, friend_id)

A friendship should be bidirectional (if (A,B) exists, (B,A) should also exist).

Task: Produce the set of missing reciprocal rows that must be inserted to make the data bidirectional, without creating duplicates.

Output columns: user_id, friend_id representing rows to insert.

Part C — Mutual friends (Python)

You are given an undirected friendship list edges, where each element is a pair (u, v) meaning u and v are friends.

Task: Implement a function that returns the set (or sorted list) of mutual friends of two users a and b.

Constraints to consider:

• The graph can be large; aim for an efficient approach.

Part D — Valid car bookings (Python)

A single car can be booked by only one customer at a time.

You are given booking requests as a list of tuples (booking_id, start_time, end_time), where start_time < end_time.

Rule: Accept bookings in increasing start_time order; a booking is valid/accepted if it does not overlap with any previously accepted booking.

Task: Return the list of accepted booking_ids.

Edge cases:

• Back-to-back bookings where end_time == next_start_time do not overlap.
• Input may be unsorted.