Calculating Timestamp Difference Between Recent 'I' Events and 'C' Event Time for Each Location

Understanding the Problem and Requirements

Overview

The given problem is a timestamp-based query that requires finding the most recent event type of ‘I’ for each location value up to the occurrence of an event type ‘C’. The goal is to calculate the timestamp difference between the ‘C’ event time and the most recent ‘I’ event time, resulting in a new table with ‘id’, ’location’, and ’timestamp_diff’ columns.

Breakdown

The problem involves several steps:

Identifying ‘C’ events.
Determining the most recent ‘I’ event for each ‘C’ event location.
Calculating the timestamp difference between the ‘C’ event time and the most recent ‘I’ event time.

Addressing Edge Cases

Background

In some cases, there may be consecutive ‘C’ events with “missing” ‘I’ events, as seen in the provided example. This can lead to incorrect results if not addressed properly.

Solution Approach

To address these edge cases, we need to modify the solution approach to ensure that all ‘C’ events are considered and the most recent ‘I’ event for each location is accurately determined.

SQL Solution

Using Window Functions

One possible way to solve this problem is by utilizing window functions in SQL. We can use the ROW_NUMBER() function to assign a unique number to each row within each partition, ordered by the event time.

WITH 
-- Assign a count of 'C' events for each id and location
`project.dataset.table` AS (
  SELECT 1001 id, TIMESTAMP '2018-06-04 18:23:48.526895 UTC' event_time, 'I' event_type, 'd' location UNION ALL
  SELECT 1001, '2018-06-04 19:26:44.359296 UTC', 'I', 'h' UNION ALL
  SELECT 1001, '2018-06-05 06:07:03.658263 UTC', 'I', 'w' UNION ALL
  SELECT 1001, '2018-06-07 00:47:44.651841 UTC', 'I', 'd' UNION ALL
  SELECT 1001, '2018-06-07 00:48:17.857729 UTC', 'C', 'd' UNION ALL
  SELECT 1001, '2018-06-08 00:04:53.086240 UTC', 'C', 'd' UNION ALL
  SELECT 1001, '2018-06-12 21:23:03.071829 UTC', 'I', 'd'
),
-- Calculate the row number of each event type for each id and location
events AS (
  SELECT id, location, event_type,
    ROW_NUMBER() OVER (PARTITION BY id, location ORDER BY event_time) AS rn,
    COUNT(CASE WHEN event_type = 'C' THEN 1 END) OVER (PARTITION BY id, location) AS c_count
  FROM `project.dataset.table`
)
-- Determine the most recent 'I' event and calculate the timestamp difference
SELECT 
  e.id, 
  e.location, 
  TIMESTAMP_DIFF(e.event_time, i.event_time, SECOND) AS diff
FROM events e
JOIN (
  SELECT id, location, MAX(event_time) AS max_event_time
  FROM `project.dataset.table`
  WHERE event_type = 'C'
  GROUP BY id, location
) c ON e.id = c.id AND e.location = c.location
JOIN (
  SELECT id, location, event_time,
    ROW_NUMBER() OVER (PARTITION BY id, location ORDER BY event_time DESC) AS rn
  FROM `project.dataset.table`
  WHERE event_type = 'I'
) i ON e.id = i.id AND e.location = i.location AND e.event_time = i.max_event_time
WHERE c.c_count > 0

Alternative Solution Using Window Functions and Case Statements

Another possible way to solve this problem is by utilizing window functions and case statements.

WITH 
-- Assign a row number for each 'I' event, ordered by the event time in descending order
i_events AS (
  SELECT id, location, event_time,
    ROW_NUMBER() OVER (PARTITION BY id, location ORDER BY event_time DESC) AS rn
  FROM `project.dataset.table`
  WHERE event_type = 'I'
),
-- Assign a count of 'C' events for each id and location
c_events AS (
  SELECT id, location, COUNT(CASE WHEN event_type = 'C' THEN 1 END) AS c_count
  FROM `project.dataset.table`
  GROUP BY id, location
)
SELECT 
  i.id, 
  i.location, 
  TIMESTAMP_DIFF(i.event_time, (
    SELECT MAX(event_time)
    FROM `project.dataset.table` t2
    WHERE t2.id = i.id AND t2.location = i.location AND t2.event_type = 'C'
  ), SECOND) AS diff
FROM i_events i
JOIN c_events c ON i.id = c.id AND i.location = c.location
WHERE c.c_count > 0

Common Issues and Solutions

Some common issues that may arise when solving this problem include:

Incorrect results due to missing ‘I’ events: To address this, make sure to use the ROW_NUMBER() function to assign a unique number to each row within each partition, ordered by the event time.
Inconsistent results due to consecutive ‘C’ events: To address this, use window functions and case statements to ensure that all ‘C’ events are considered and the most recent ‘I’ event for each location is accurately determined.

By following these steps and using the provided SQL solutions, you should be able to accurately calculate the timestamp difference between the ‘C’ event time and the most recent ‘I’ event time.

Last modified on 2025-04-16