A walkthrough of the solution to SQL Murder Mystery by Northwestern University Knight Lab. Solution by Mikhail Popov (@bearloga)

sql-murder-mystery-solution.md

Solution to SQL Murdery Mystery

A walkthrough of the solution to SQL Murder Mystery by Northwestern University Knight Lab. Solution by Mikhail Popov

Prompt

A crime has taken place and the detective needs your help. The detective gave you the crime scene report, but you somehow lost it. You vaguely remember that the crime was a ​murder​ that occurred sometime on ​Jan.15, 2018​ and that it took place in ​SQL City​. Start by retrieving the corresponding crime scene report from the police department’s database.

Witness reports

SELECT description FROM crime_scene_report
WHERE date = '20180115' AND type = 'murder' AND city = 'SQL City'

Security footage shows that there were 2 witnesses. The first witness lives at the last house on "Northwestern Dr". The second witness, named Annabel, lives somewhere on "Franklin Ave".

WITH witness1 AS (
    SELECT id FROM person
    WHERE address_street_name = 'Northwestern Dr'
    ORDER BY address_number DESC LIMIT 1
), witness2 AS (
    SELECT id FROM person
    WHERE INSTR(name, 'Annabel') > 0 AND address_street_name = 'Franklin Ave'
), witnesses AS (
    SELECT *, 1 AS witness FROM witness1
    UNION
    SELECT *, 2 AS witness FROM witness2
)
SELECT witness, transcript FROM witnesses
LEFT JOIN interview ON witnesses.id = interview.person_id

Witness 1

I heard a gunshot and then saw a man run out. He had a "Get Fit Now Gym" bag. The membership number on the bag started with "48Z". Only gold members have those bags. The man got into a car with a plate that included "H42W".

Witness 2

I saw the murder happen, and I recognized the killer from my gym when I was working out last week on January the 9th.

Suspects

Cross-referencing the gym memberships, gym check-ins, and drivers license:

WITH gym_checkins AS (
    SELECT person_id, name
    FROM get_fit_now_member
    LEFT JOIN get_fit_now_check_in ON get_fit_now_member.id = get_fit_now_check_in.membership_id
    WHERE membership_status = 'gold' -- Only gold members have those bags
      AND id REGEXP '^48Z' -- membership number on the bag started with "48Z"
      AND check_in_date = '20180109' -- Witness 2 recognized him on January the 9th
), suspects AS (
    SELECT gym_checkins.person_id, gym_checkins.name, plate_number, gender
    FROM gym_checkins
    LEFT JOIN person ON gym_checkins.person_id = person.id
    LEFT JOIN drivers_license ON person.license_id = drivers_license.id
)
SELECT * FROM suspects
-- The man got into a car with a plate that included "H42W"
WHERE INSTR(plate_number, 'H42W') > 0 AND gender = 'male'

person_id	name	plate_number	gender
67318	Jeremy Bowers	0H42W2	male

INSERT INTO solution VALUES (1, "Jeremy Bowers");

SELECT value FROM solution;

Congrats, you found the murderer! But wait, there's more... If you think you're up for a challenge, try querying the interview transcript of the murderer to find the real villian behind this crime. If you feel especially confident in your SQL skills, try to complete this final step with no more than 2 queries.

Further investigation

SELECT transcript FROM interview WHERE person_id = 67318

I was hired by a woman with a lot of money. I don't know her name but I know she's around 5'5" (65") or 5'7" (67"). She has red hair and she drives a Tesla Model S. I know that she attended the SQL Symphony Concert 3 times in December 2017.

WITH red_haired_tesla_drivers AS (
    SELECT id AS license_id
    FROM drivers_license
    WHERE gender = 'female' AND hair_color = 'red' -- She has red hair
      AND car_make = 'Tesla' AND car_model = 'Model S' -- and she drives a Tesla Model S
      AND height >= 64 AND height <= 68 -- she's around 5'5" (65") or 5'7" (67")
), rich_suspects AS (
    SELECT person.id AS person_id, name, annual_income
    FROM red_haired_tesla_drivers AS rhtd
    LEFT JOIN person ON rhtd.license_id = person.license_id
    LEFT JOIN income ON person.ssn = income.ssn
), symphony_attenders AS (
    SELECT person_id, COUNT(1) AS n_checkins
    FROM facebook_event_checkin
    WHERE event_name = 'SQL Symphony Concert' -- she attended the SQL Symphony Concert
      AND `date` REGEXP '^201712' -- in December 2017
    GROUP BY person_id
    HAVING n_checkins = 3 -- 3 times
)
SELECT name, annual_income
FROM rich_suspects
INNER JOIN symphony_attenders ON rich_suspects.person_id = symphony_attenders.person_id

name	annual_income
Miranda Priestly	310000

INSERT INTO solution VALUES (1, "Miranda Priestly");

SELECT value FROM solution;

Congrats, you found the brains behind the murder! Everyone in SQL City hails you as the greatest SQL detective of all time. Time to break out the champagne!

this is tough man