Are you familiar with the concept of partitions and how PARTITION BY with ORDER BY is used?

The concept of partitions in SQL is used to divide a dataset into logical groups on which window functions can be applied. The keyword PARTITION BY determines how to split the data into these groups.

ORDER BY within a window function sets the order of rows within each partition, which is important for functions that depend on order, such as ROW_NUMBER(), LAG(), LEAD(), etc.

Example:

SELECT
  user_id,
  event_time,
  ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY event_time) AS event_rank
FROM events;

Here, data is partitioned by each user (user_id), and within each partition, events are sorted by time to number them in chronological order.