Home > Database > Mysql Tutorial > How Can I Find the Next Activity from Group B Following Group A Activities in PostgreSQL Using Window Functions?

How Can I Find the Next Activity from Group B Following Group A Activities in PostgreSQL Using Window Functions?

Barbara Streisand
Release: 2024-12-25 16:54:17
Original
540 people have browsed it

How Can I Find the Next Activity from Group B Following Group A Activities in PostgreSQL Using Window Functions?

Conditional Lead/Lag Function in PostgreSQL

Your task is to generate a query that retrieves specific activity sequences for users from a given table. You want to determine the next activity from group B (that always occurs after a group A activity) for each user.

Problem Definition

Consider the following table:

Name activity time
user1 A1 12:00
user1 E3 12:01
user1 A2 12:02
user2 A1 10:05
user2 A2 10:06
user2 A3 10:07
user2 M6 10:07
user2 B1 10:08
user3 A1 14:15
user3 B2 14:20
user3 D1 14:25
user3 D2 14:30

The desired output for this table is:

Name activity next_activity
user1 A2 NULL
user2 A3 B1
user3 A1 B2

Solution

You can solve this problem by leveraging the DISTINCT ON and CASE statements in conjunction with window functions:

SELECT name
     , CASE WHEN a2 LIKE 'B%' THEN a1 ELSE a2 END AS activity
     , CASE WHEN a2 LIKE 'B%' THEN a2 END AS next_activity
FROM  (
   SELECT DISTINCT ON (name)
          name
        , lead(activity) OVER (PARTITION BY name ORDER BY time DESC) AS a1
        , activity AS a2
   FROM   t
   WHERE (activity LIKE 'A%' OR activity LIKE 'B%')
   ORDER  BY name, time DESC
   ) sub;
Copy after login

Explanation

  • The subquery identifies the latest activity from group A and the following activity from group B (if any) for each user using the DISTINCT ON and window function lead() with an ORDER BY time DESC.
  • The CASE statements handle the desired output: the latest activity from group A, and the next activity from group B (if it exists).

Conditional Window Functions

While PostgreSQL does not support conditional window functions directly (e.g., lead(activity) FILTER (WHERE activity LIKE 'A%')), you can utilize the FILTER clause with aggregate functions and use them as window functions:

lead(activity) FILTER (WHERE activity LIKE 'A%') OVER () AS activity
Copy after login

However, this approach is inefficient and impractical for large datasets. Instead, the solution presented above is recommended for both small and large datasets.

The above is the detailed content of How Can I Find the Next Activity from Group B Following Group A Activities in PostgreSQL Using Window Functions?. For more information, please follow other related articles on the PHP Chinese website!

source:php.cn
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Latest Articles by Author
Popular Tutorials
More>
Latest Downloads
More>
Web Effects
Website Source Code
Website Materials
Front End Template