LAG Examples in SQL

What LAG does

Return the previous row value. SQL syntax can vary by database, but the pattern below is a useful starting point for reports and analysis.

Syntax or pattern

LAG(amount) OVER (PARTITION BY customer_id ORDER BY order_date)

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5 practical examples

Use LAG in a sales report

Apply the LAG pattern to a sales table.

-- LAG example for sales
SELECT customer_id, order_date, total_amount
FROM orders
WHERE total_amount > 100;

This shows how the LAG pattern can support a simple sales analysis.

Use LAG for customers

Apply the LAG pattern to customer records.

-- LAG 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 LAG for products

Apply the LAG pattern to product or inventory data.

-- LAG example for products
SELECT product_id, product_name, category
FROM products;

Product tables are good practice data for this SQL pattern.

Use LAG for monthly reporting

Apply the LAG pattern to a monthly reporting query.

-- LAG 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 LAG during data checks

Apply the LAG pattern to find data quality issues.

-- LAG 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.

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