SQL ROW_NUMBER

What is SQL ROW_NUMBER and how do you use it?

SQL ROW_NUMBER assigns a unique, sequential integer to each row within a result-set partition, ordered by a specified column list.

Description

What Is SQL ROW_NUMBER?

ROW_NUMBER is a window function that returns a sequential integer, starting at 1, to each row in a result set. Numbers restart when a PARTITION BY clause is present. Because ranking happens after the FROM, WHERE, and GROUP BY stages, ROW_NUMBER can reference aggregated or joined columns.

How Does ROW_NUMBER Work in Major Databases?

ROW_NUMBER syntax is ANSI-SQL and works in PostgreSQL, SQL Server, Snowflake, BigQuery, Redshift, and Oracle. You write ROW_NUMBER() OVER (PARTITION BY ... ORDER BY ...). If PARTITION BY is omitted, the numbering spans the entire query result.

What Problems Does ROW_NUMBER Solve?

ROW_NUMBER eliminates self-joins and subqueries when deduplicating, paginating results, or selecting the first/last row per group. It improves readability and often boosts performance compared to correlated subqueries.

How Do I Write a Basic ROW_NUMBER Query?

Place the function in the SELECT list, define ORDER BY for deterministic numbering, and optionally partition.SELECT user_id, created_at, ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn FROM events;

How Do I Reset the Counter with PARTITION BY?

Add a PARTITION BY clause listing columns that define each group. The sequence restarts at 1 for every distinct combination, letting you rank rows within categories like customer, date, or region.

How Do I Filter to Top N Rows per Group?

Wrap the ROW_NUMBER query in a CTE or subquery, then filter on the generated column.WITH ranked AS ( SELECT *, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY total DESC) AS rn FROM orders) SELECT * FROM ranked WHERE rn <= 3;

How Do I Deduplicate Rows with ROW_NUMBER?

Generate ROW_NUMBER ordered by the field that defines “most recent” or “preferred.” Delete or ignore rows where rn > 1, keeping a single record per key.

What Are Best Practices for ROW_NUMBER?

Always include deterministic columns in ORDER BY to avoid non-repeatable ranks. Use indexes on ORDER BY fields for large tables. In OLTP systems, run ROW_NUMBER in read replicas to offload work.

What Are Common Pitfalls with ROW_NUMBER?

Forgetting ORDER BY causes arbitrary numbering. Using ROW_NUMBER instead of RANK or DENSE_RANK when ties matter yields missing ranks. Filtering on ROW_NUMBER in the same SELECT without a subquery can fail in some dialects.

Why SQL ROW_NUMBER is important

ROW_NUMBER simplifies complex ranking logic into a single, readable clause, reducing query length and execution time. Because it is ANSI-standard, knowledge transfers across platforms, making engineers more versatile. In analytics pipelines, ROW_NUMBER is the go-to tool for de-duping event streams and building slowly changing dimensions without procedural code. ROW_NUMBER also underpins pagination in APIs and dashboards, letting developers return consistent slices of large datasets. Mastery of ROW_NUMBER therefore accelerates both backend engineering and user-facing analytics.

SQL ROW_NUMBER Example Usage


Find the most recent login per user:

```sql
WITH sessions AS (
  SELECT
    user_id,
    login_time,
    ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY login_time DESC) AS rn
  FROM user_logins)
SELECT user_id, login_time
FROM sessions
WHERE rn = 1;
```

SQL ROW_NUMBER Syntax

Common Mistakes

Omitting ORDER BY, which results in non-deterministic numbering. Fix: Always specify columns in ORDER BY that guarantee a stable sort, such as timestamps or surrogate keys.
Filtering on ROW_NUMBER in the same SELECT layer (e.g., WHERE rn = 1) in databases that disallow alias referencing. Fix: Wrap the query in a CTE or subquery, then filter.
Using ROW_NUMBER instead of DENSE_RANK when tie handling matters, causing gaps in rankings. Fix: Choose DENSE_RANK for contiguous numbering when ties should share a rank.

Frequently Asked Questions (FAQs)

Is ROW_NUMBER the same as RANK?

No. RANK creates gaps after ties, DENSE_RANK keeps ranks contiguous, and ROW_NUMBER assigns a unique value even when rows tie.

Can I paginate results with ROW_NUMBER?

Yes. Number all rows, then filter for a numeric range—for example, rows 51–100. This works in any database supporting ROW_NUMBER.

How does Galaxy help with ROW_NUMBER queries?

Galaxy’s AI copilot autocompletes the OVER clause, suggests correct ORDER BY columns, and previews the numbered result, speeding up query writing.

What is the performance impact of ROW_NUMBER?

ROW_NUMBER requires sorting. Indexes on ORDER BY columns and partition pruning keep the operation efficient even on large datasets.