22 problems where the obvious schema breaks.
Six tracks, in the order they are worth learning. Each one is a business scenario with rules that pull against each other just enough to matter.
Track 1
Fundamentals
6 exercises, 2 free
0 of 6 solved
- Free
The two constraints every table needs
Required fields and uniqueness: the first two rules a table enforces, and what gets in the moment it does not.
Difficulty 1 of 5 - Free
The order with no customer
An order has to belong to a customer who exists, a customer can have many orders, and the database should be the one enforcing both.
Difficulty 1 of 5 - Subscribers
The key the business insists on renaming
Warehouse codes look like perfect primary keys until operations renames one and every shipment points at nothing.
Difficulty 2 of 5 - Subscribers
Columns that refuse what should never be stored
A rating, an amount of money and a timestamp: three columns where the obvious type quietly accepts wrong data.
Difficulty 2 of 5 - Subscribers
Exactly one of them is the main one
A person keeps several addresses, at most one is primary, and a boolean column will not enforce that.
Difficulty 3 of 5 - Subscribers
A category tree with no way back to the root
A parent_id column makes a tree, and nothing about it stops a category from becoming its own grandparent.
Difficulty 4 of 5
Track 2
Deletion & History
4 exercises, 0 free
0 of 4 solved
- Subscribers
Soft delete without breaking sign-ups
Keep deleted accounts on record forever, and still let someone sign up again with the same email.
Difficulty 3 of 5 - Subscribers
A record of who changed what, that nobody can change
Log every salary change automatically, and make the log itself refuse to be edited or deleted.
Difficulty 4 of 5 - Subscribers
Every draft kept, exactly one of them live
Documents keep every revision, version numbers restart at 1 for each document, and only one version is ever published.
Difficulty 4 of 5 - Subscribers
A balance that cannot be wrong
Money moves between accounts, never appears, and the balance is something you derive rather than something you set.
Difficulty 5 of 5
Track 3
Multi-tenancy
3 exercises, 0 free
0 of 3 solved
- Subscribers
Uniqueness that stops at the tenant boundary
The same email has to be free to join two different companies, and blocked from joining one of them twice.
Difficulty 2 of 5 - Subscribers
Isolation that survives a forgotten WHERE clause
One table holds every tenant’s invoices, and a query with no tenant filter must still see only its own.
Difficulty 5 of 5 - Subscribers
Invoice numbers that start at 1 for every customer
Each tenant numbers its own invoices from 1, with no gaps, and a database sequence can do neither.
Difficulty 4 of 5
Track 4
Time
3 exercises, 0 free
0 of 3 solved
- Subscribers
A price that changes without erasing what it was
Answer what a product cost on any past date, instead of losing that answer every time the price is updated.
Difficulty 3 of 5 - Subscribers
The day it happened depends on where you are standing
A booking has an instant and a local day, and storing both as the same type gets one of them wrong.
Difficulty 4 of 5 - Subscribers
Two people cannot hold the same desk at once
Assignments have a start and an end, and no two for the same desk may overlap in time.
Difficulty 5 of 5
Track 5
Normalization
3 exercises, 0 free
0 of 3 solved
- Subscribers
Tags that a search can actually trust
A comma-separated column looks like tags until searching for "sql" also returns everything tagged "nosql".
Difficulty 3 of 5 - Subscribers
The invoice that must not change when the price does
A shop raises a price and last month’s orders have to stay exactly what the customer was charged.
Difficulty 4 of 5 - Subscribers
One column must be copied, the other must not
A shipment freezes the address it went to and follows the customer’s email wherever it changes: same table, opposite rules.
Difficulty 4 of 5
Track 6
Performance
3 exercises, 0 free
0 of 3 solved
- Subscribers
An index serves a question, not a column
Indexing each column separately feels thorough and still leaves the one query that matters sorting the whole table.
Difficulty 3 of 5 - Subscribers
The column is indexed and the query still scans
Wrapping a column in lower() hides it from its own index, and the plan looks the same as having no index at all.
Difficulty 4 of 5 - Subscribers
A number that is right and cheap at the same time
The comment count on every thread, without counting comments, and still correct after a delete.
Difficulty 5 of 5