What is a window function?

A window function performs a calculation across a set of rows related to the current row (the "window") without collapsing them into a single group row. Unlike GROUP BY aggregates, each input row keeps its own output row.

What is the difference between RANK and DENSE_RANK?

RANK skips numbers after a tie — two rows tied at rank 2 are both rank 2, and the next row is rank 4. DENSE_RANK never skips — the next rank after two 2s is 3.

When should I use a frame clause?

Frame clauses (ROWS / RANGE BETWEEN) are needed for running totals, moving averages, or any aggregate that should only look at a sliding subset of the partition. Without a frame, aggregate window functions default to RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.

Does LAG / LEAD work the same in all databases?

The syntax is identical in PostgreSQL, MySQL 8+, SQL Server 2012+, and SQLite 3.25+. The offset defaults to 1 and the default value defaults to NULL if omitted — this tool makes both explicit for clarity.

Can I use multiple window functions in the same query?

Yes. You can apply as many window functions as you need in the SELECT list. If two functions share the same OVER clause, SQL Server and PostgreSQL let you name the window with a WINDOW alias to avoid repetition.

Which databases support window functions?

PostgreSQL 8.4+, MySQL 8.0+, SQL Server 2005+ (basic) / 2012+ (full), Oracle 8i+, SQLite 3.25+, MariaDB 10.2+, and BigQuery / Snowflake / Redshift all support the standard syntax shown by this tool.

Window Function Helper

Name: Window Function Helper
Availability: InStock
Author: Nham Vu

Interactive reference and syntax builder for SQL window functions including ROW_NUMBER, RANK, LAG, LEAD, and aggregate OVER clauses.

Build Your Window Function

Function

Column / Expression

PARTITION BY (comma-separated, leave blank for no partition)

ORDER BY (comma-separated)

Dir

Output Column Alias

Generated Syntax

-- Fill in the fields on the left and click Generate SQL

Function Reference

Function	Returns	ORDER BY required?

Sample Output Preview

Based on example employee table

dept	name	salary	result

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Summary

Interactive reference and syntax builder for SQL window functions including ROW_NUMBER, RANK, LAG, LEAD, and aggregate OVER clauses.

How it works

Select a window function from the dropdown (ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, SUM, AVG, etc.).
Enter your PARTITION BY column(s) — leave blank to partition over the entire result set.
Enter your ORDER BY column(s) and pick a direction (ASC / DESC).
For LAG / LEAD, set the offset and optional default value.
For aggregate functions, choose a frame clause (ROWS BETWEEN, RANGE BETWEEN).
Click "Generate SQL" to get ready-to-paste syntax with a sample result preview.

Use cases

Build ranking queries (top-N per group) without a self-join.
Calculate running totals or moving averages with SUM / AVG OVER.
Compare a row to the previous or next row using LAG / LEAD.
Assign row numbers within partitions for pagination or deduplication.
Compute percentile ranks and cumulative distributions with PERCENT_RANK.
Write interview-ready window function queries on the fly.
Learn the difference between RANK and DENSE_RANK with side-by-side output.
Prototype frame clauses for rolling 7-day windows before running on a database.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu