SQL pattern · P09 · 13 problems

Set operations

Graded problems on set operations, from easy to hard.

13 problems · about 2 h in all
Start with problem 01Combine Web and Mobile Events · 4 min
Learn it first · P09The UNION that quietly drops a rowUNION removes duplicate rows; UNION ALL keeps every meaningful record.Read the lesson →

Easy

3 problems · about 14 min
01Combine Web and Mobile Events4 min →02Customers in Both BI and Finance5 min →03All Distinct Customers Across Both Systems5 min →

Medium

6 problems · about 46 min
04Users Active on Web and Mobile on the Same Day8 min →05Customers Active in Both Halves of May8 min →06Total Transactions Across Two Payment Processors6 min →07Customers in BI but Not Finance6 min →08Customers in Exactly One System8 min →09Surplus Copies Between Two Tables10 min →

Hard

4 problems · about 58 min
10Reconcile BI and Finance Customer Counts12 min →11Eligible Customers After All Exclusion Lists10 min →12Which Systems Each Customer Belongs To11 min →13Minimum Changes Between Two Keyless Pricebook Releases25 min →
Up next · P10

The subquery that answers the wrong question

15 practice problems. A missing correlation compares each row to the overall average, not its own group.

Practise P10Read P10