← SQL Operations Cases
Lesson 4 of 4
Diagnose duplicate customer keys
Find shared nonblank emails without deleting records.
Do the work
- Two customer IDs may share an email.
- Exclude NULL and empty text from this diagnostic.
- Report the duplicate key and its count.
See the method
Avery and Morgan share team@example.test.
Read every demonstration step
- Avery and Morgan share team@example.test.
- Group by email and filter COUNT(*) > 1.
- Keep the original customer records unchanged.
Practice independently
Exercise 1
Find shared customer emails
Open practice →Exercise 2Find paid orders with unmatched customer IDs
Open practice →Take it further
A shared email can be legitimate; flag it for review instead of merging automatically.