What Find Duplicates does
Find duplicate emails, IDs or records. SQL syntax can vary by database, but the pattern below is a useful starting point for reports and analysis.
Syntax or pattern
SELECT email, COUNT(*) FROM customers GROUP BY email HAVING COUNT(*) > 1;5 practical examples
Use Find Duplicates in a sales report
Apply the Find Duplicates pattern to a sales table.
-- Find Duplicates example for sales
SELECT customer_id, order_date, total_amount
FROM orders
WHERE total_amount > 100;This shows how the Find Duplicates pattern can support a simple sales analysis.
Use Find Duplicates for customers
Apply the Find Duplicates pattern to customer records.
-- Find Duplicates example for customers
SELECT customer_id, email, status
FROM customers
WHERE status = 'Active';This is useful when customer records need filtering, labeling or summarizing.
Use Find Duplicates for products
Apply the Find Duplicates pattern to product or inventory data.
-- Find Duplicates example for products
SELECT product_id, product_name, category
FROM products;Product tables are good practice data for this SQL pattern.
Use Find Duplicates for monthly reporting
Apply the Find Duplicates pattern to a monthly reporting query.
-- Find Duplicates example for monthly reporting
SELECT DATE_TRUNC('month', order_date) AS month, SUM(total_amount) AS sales
FROM orders
GROUP BY DATE_TRUNC('month', order_date);This turns row-level transactions into a report-friendly result.
Use Find Duplicates during data checks
Apply the Find Duplicates pattern to find data quality issues.
-- Find Duplicates example for data checks
SELECT customer_id, COUNT(*) AS records
FROM orders
GROUP BY customer_id
HAVING COUNT(*) > 1;This is a useful pattern for auditing data before building a report.
Common mistakes to avoid
- Forgetting that SQL dialects vary across PostgreSQL, SQL Server, MySQL, BigQuery and SQLite.
- Using SELECT * in production reports when only a few columns are needed.
- Not checking join keys, duplicate rows or NULL values before trusting results.
FAQ
Will this SQL work in every database?
The idea is portable, but function names and date syntax may vary. Check your database dialect if a function is not recognized.
Should I use this in a report query?
Yes, if the pattern matches the business question and you have checked filters, joins and row counts.
Why does my result have too many rows?
The most common reasons are duplicate join keys, missing filters or grouping at the wrong level of detail.