TOPN

Description

The TOPN function returns the N most frequent values in the specified column. It is an approximate calculation function that returns results ordered by count in descending order.

Syntax

TOPN(<expr>, <top_num> [, <space_expand_rate>])

Parameters

Parameter	Description
<expr>	The column or expression to be counted. Supported types: TinyInt, SmallInt, Integer, BigInt, LargeInt, Float, Double, Decimal, Date, Datetime, IPV4, IPV6, String.
<top_num>	The number of most frequent values to return. Must be a positive INT constant.
<space_expand_rate>	Optional INT constant, default 50. A positive value retains at most top_num * space_expand_rate candidates when serializing intermediate state; larger values generally improve accuracy. This is not a hard limit on local aggregation memory. In the Doris 4.1 series since 4.1.5, a value of 0 or less retains all intermediate candidates, while the final result still contains at most top_num values. This mode may use more memory and remains subject to query memory limits.

Return Value

Returns a JSON string containing the values and their counts. If there is no valid data in the group, returns NULL.

Example

-- setup
CREATE TABLE page_visits (
    page_id INT,
    user_id INT,
    visit_date DATE
) DISTRIBUTED BY HASH(page_id)
PROPERTIES (
    "replication_num" = "1"
);
INSERT INTO page_visits VALUES
(1, 101, '2024-01-01'),
(2, 102, '2024-01-01'),
(1, 103, '2024-01-01'),
(3, 101, '2024-01-01'),
(1, 104, '2024-01-01'),
(2, 105, '2024-01-01'),
(1, 106, '2024-01-01'),
(4, 107, '2024-01-01');

SELECT TOPN(page_id, 3) as top_pages
FROM page_visits;

Find the top 3 most visited pages.

+---------------------+
| top_pages           |
+---------------------+
| {"1":4,"2":2,"4":1} |
+---------------------+

SELECT TOPN(page_id, 3) as top_pages
FROM page_visits where page_id is null;

+-----------+
| top_pages |
+-----------+
| NULL      |
+-----------+

Retain all intermediate candidates, but return only the most frequent value:

SELECT TOPN(s, 1, 0)
FROM (SELECT 'a' AS s UNION ALL SELECT 'b' UNION ALL SELECT 'a') t;

{"a":2}