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Pattern

Filtering

12 problemsin this pattern, climbing easy → medium → hard. Pick wherever you're calibrated.

EZ

Easy

0 / 5 solved

Filter to paid orders
SQL · ~3 min
NEW
Active premium customers
SQL · ~4 min
NEW
Customers with missing phone numbers
SQL · ~4 min
NEW
Customers in a set of countries
SQL · ~4 min
NEW
Products within a price band
SQL · ~4 min
NEW

MD

Medium

0 / 5 solved

Customers with at least one order
SQL · ~7 min
NEW
Open orders (including NULL status)
SQL · ~7 min
NEW
Users active in the last 24 hours
SQL · ~7 min
NEW
Orders that aren't paid
SQL · ~7 min
NEW
Premium customers we can call
SQL · ~6 min
NEW

HD

Hard

0 / 2 solved

Customers not on the blocklist (NOT IN NULL trap)
SQL · ~12 min
NEW
Outer join + date filter — predicate placement
SQL · ~15 min
NEW