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.
<top_num>	The number of the most frequent values to return. It must be a positive integer.
<space_expand_rate>	Optional parameter, which is used to set the number of counters used in the Space-Saving algorithm. counter_numbers = top_num * space_expand_rate , the larger the value of space_expand_rate, the more accurate the result, and the default value is 50.

Return Value

Returns a JSON string containing values and their corresponding occurrence counts.

Examples

-- Create sample table
CREATE TABLE page_visits (
    page_id INT,
    user_id INT,
    visit_date DATE
) DISTRIBUTED BY HASH(page_id)
PROPERTIES (
    "replication_num" = "1"
);

-- Insert test data
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');

-- Find top 3 most visited pages
SELECT TOPN(page_id, 3) as top_pages
FROM page_visits;

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