The Laundry Mark

BLOOMSBURY, LONDON · DIFFICULTY: APPRENTICE

THE BRIEFING

Someone went through the back window of a house on Doughty Street and left in a hurry when the dog started. They left a coat on the railings. There is no name in the coat. Sewn inside the collar is a laundry mark, the code a laundry stitches in so it can give the right garment back to the right customer. The mark reads H-447. The laundry keeps two books. One says which mark belongs to which customer number. The other says which customer number belongs to which person. Neither book on its own gives you a name. This is what JOIN is for. It follows the customer number out of the first table and into the second, so you can read the mark and the name in the same row.

THIS CASE TEACHES

JOIN. How to follow a key from one table into another.

FOUND IN THE COAT

LAUNDRY MARK, STITCHED INSIDE COLLAR

        H-447

Blue thread, machine stitched, worn at the edges.
The garment is a heavy overcoat, dark grey, one
button missing from the cuff.

CLUES ON THE TABLE

01.The mark sewn into the collar is H-447. Read it exactly: there is also an H-44 and an H-474 in the book.
02.Both tables carry a column called customer_id. That shared column is what lets you join them.
03.The shape is: FROM laundry_marks JOIN customers ON laundry_marks.customer_id = customers.customer_id.
04.You can do it in two separate queries instead. The JOIN is the same two steps written as one.

/ THE EVIDENCE

Database Schema

These are the tables you can query. The lines are the keys that join them. Hover a table to see what it connects to.

laundry_marks3 COL

mark_codeVARCHAR(10)
stitched inside the collar
customer_idINT
→ customers.customer_id
registered_atDATE

customers4 COL

customer_idINT
nameVARCHAR(60)
streetVARCHAR(80)
account_openedDATE

JOINS TO ANOTHER TABLEJOINED FROM ANOTHER TABLE

/ THE QUERY TERMINAL

Run your queries

Use SQL to explore the data. Each table holds part of the answer.

query.sql · 0 queries

or Ctrl+Enter

/ NOTES

Your notepad

Write down anything useful as you go.

NOTES

Your notes and your last query are kept in this browser.

/ THE ACCUSATION

Name your suspect

Type the suspect's full name and submit.

NEXT CASE

The Cab Ranks →

"Somebody watched that house for a week. Nobody remembers a face. The cab book remembers every fare."