The Difference Between ROW_NUMBER(), RANK(), and DENSE_RANK()

Posted on by lukaseder

One of the best features in SQL are window functions. Dimitri Fontaine put it bluntly:

There was SQL before window functions and SQL after window functions

If you’re lucky enough to be using any of these databases, then you can use window functions yourself:

  • CUBRID
  • DB2
  • Firebird
  • H2
  • Informix
  • MariaDB
  • MySQL
  • Oracle
  • PostgreSQL
  • SQLite
  • SQL Server
  • Sybase SQL Anywhere
  • Teradata

(source here)

One of the most obvious and useful set of window functions are ranking functions where rows from your result set are ranked according to a certain scheme. There are three ranking functions:

  • ROW_NUMBER()
  • RANK()
  • DENSE_RANK()

The difference is easy to remember. For the examples, let’s assume we have this table (using PostgreSQL syntax):

CREATE TABLE t(v) AS
SELECT * FROM (
  VALUES('a'),('a'),('a'),('b'),
        ('c'),('c'),('d'),('e')
) t(v)

ROW_NUMBER()

… assigns unique numbers to each row within the PARTITION given the ORDER BY clause. So you’d get:

SELECT v, ROW_NUMBER() OVER()
FROM t

Note that some SQL dialects (e.g. SQL Server) require an explicit ORDER BY clause in the OVER() clause:

SELECT v, ROW_NUMBER() OVER(ORDER BY v)
FROM t

The above query returns:

| V | ROW_NUMBER |
|---|------------|
| a |          1 |
| a |          2 |
| a |          3 |
| b |          4 |
| c |          5 |
| c |          6 |
| d |          7 |
| e |          8 |

RANK()

… behaves like ROW_NUMBER(), except that “equal” rows are ranked the same. If we substitute RANK() into our previous query:

SELECT v, RANK() OVER(ORDER BY v)
FROM t

… then the result we’re getting is this:

| V | RANK |
|---|------|
| a |    1 |
| a |    1 |
| a |    1 |
| b |    4 |
| c |    5 |
| c |    5 |
| d |    7 |
| e |    8 |

As you can see, much like in a sports ranking, we have gaps between the different ranks. We can avoid those gaps by using

DENSE_RANK()

Trivially, DENSE_RANK() is a rank with no gaps, i.e. it is “dense”. We can write:

SELECT v, DENSE_RANK() OVER(ORDER BY v)
FROM t

… to obtain

| V | DENSE_RANK |
|---|------------|
| a |          1 |
| a |          1 |
| a |          1 |
| b |          2 |
| c |          3 |
| c |          3 |
| d |          4 |
| e |          5 |

One interesting aspect of DENSE_RANK() is the fact that it “behaves like” ROW_NUMBER() when we add the DISTINCT keyword.

SELECT DISTINCT v, DENSE_RANK() OVER(ORDER BY v)
FROM t

… to obtain

| V | DENSE_RANK |
|---|------------|
| a |          1 |
| b |          2 |
| e |          5 |
| d |          4 |
| c |          3 |

In fact, ROW_NUMBER() prevents you from using DISTINCT, because ROW_NUMBER() generates unique values across the partition before DISTINCT is applied:

SELECT DISTINCT v, ROW_NUMBER() OVER(ORDER BY v)
FROM t
ORDER BY 1, 2

DISTINCT has no effect:

| V | ROW_NUMBER |
|---|------------|
| a |          1 |
| a |          2 |
| a |          3 |
| b |          4 |
| c |          5 |
| c |          6 |
| d |          7 |
| e |          8 |

Putting it all together

A good way to understand the three ranking functions is to see them all in action side-by-side. Run this query

SELECT 
  v, 
  ROW_NUMBER() OVER(ORDER BY v),
  RANK()       OVER(ORDER BY v),
  DENSE_RANK() OVER(ORDER BY v)
FROM t
ORDER BY 1, 2

… or this one (using the SQL standard WINDOW clause, to reuse window specifications):

SELECT 
  v, 
  ROW_NUMBER() OVER(w),
  RANK()       OVER(w),
  DENSE_RANK() OVER(w)
FROM t
WINDOW w AS (ORDER BY v)

… to obtain:

| V | ROW_NUMBER | RANK | DENSE_RANK |
|---|------------|------|------------|
| a |          1 |    1 |          1 |
| a |          2 |    1 |          1 |
| a |          3 |    1 |          1 |
| b |          4 |    4 |          2 |
| c |          5 |    5 |          3 |
| c |          6 |    5 |          3 |
| d |          7 |    7 |          4 |
| e |          8 |    8 |          5 |

Note that unfortunately, the WINDOW clause is not supported in all databases.

SQL is awesome

These things can be written very easily using SQL window functions. Once you get a hang of the syntax, you won’t want to miss this killer feature in your every day SQL statements any more. Excited?

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26 thoughts on “The Difference Between ROW_NUMBER(), RANK(), and DENSE_RANK()”

  1. Great explanation. Window functions have many operations and it’s easy to get lost when you start working with them. I always like to remind Java developers that Hibernate query language is no match for window functions, pivot or recursive common table expressions. It’s pretty sad to find out that the vast majority of Java developers have never heard of these SQL features.

    Reply

  6. How to use ROW_NUMBER or similar function in Firebird? Need to have current row number in SELECT.

    Reply

    1. You wrap them in a derived table and query on that:

      SELECT *
FROM (
  SELECT row_number() OVER (...) rn,
  FROM ...
) t
WHERE t.rn IN (2, 3)
      Reply
  8. SELECT 
CASE
  WHEN ((DENSE_RANK() OVER(ORDER BY v) ) ) = 2 THEN 'Rank value'||' '||(DENSE_RANK() OVER(ORDER BY v) )||' '||'col value'||v
END
FROM t;
----The above gives 2nd highest rank
    Reply

    2. Not sure which server this is intended for but when I run this I am getting the following error.
      Incorrect syntax near ‘|’.

      Reply

      1. There’s a small remark in the middle of the article: “(using PostgreSQL syntax)”. Your error message suggests you may have executed a result set, because there is no pipe symbol in any SQL statement… :P

        Reply

  9. how to avoid duplicates here: 

    SELECT A.NR, A.NAME, B.NR FROM A INNER JOIN B ON A.NR=B.NR WHERE B.NR IN (25, 26, 27)
    Reply

      1. Hi, if I have a column of indicator, say, it is like 00100100001000001111, what I hope to do is that when the SQL sees 1, it will automatically increase the index by 1, that is to say, in this example, we will end up with 111222333334444445678, something like this.

        Any idea of how to use SQL do this?

        Best,
        Felix

        Reply

  13. You may want to update your list of DBs with Window Functions since MariaDB now has window functions.

    Reply

  14. Thanks

    I was struggling to deal with a situation of repeated records and after seeing the example with dense_rank helped me to find my solution (dense_rank() over (partition by (…) )

    Reply

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