Answer
Duplicate values can be found by grouping the candidate key columns and keeping groups with more than one row. • GROUP BY defines which values count as duplicates. • COUNT(*) measures each group size. • Include every column that belongs to the duplicate definition.
💡 SQL Example
SELECT email, COUNT(*) AS copies FROM users GROUP BY email HAVING COUNT(*) > 1;
Result
Repeated email values and their counts
⚡ Quick Revision
GROUP BY the duplicate key and use HAVING COUNT(*) > 1.