How can I obtain the difference between time-based data points in MySQL when they do not have consecutive IDs?

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There are several ways to obtain the difference between time-based data points in MySQL when they do not have consecutive IDs. Here are three options:

1. Use the TIMESTAMPDIFF() function:

If your data has timestamps, you can use the TIMESTAMPDIFF() function to calculate the difference between two timestamps. Here's an example of how to do that:

SELECT TIMESTAMPDIFF(MINUTE, t1.timestamp, t2.timestamp) AS elapsed_time
FROM my_table t1
JOIN my_table t2 ON t2.timestamp > t1.timestamp
WHERE t2.id > t1.id
ORDER BY t1.id;

This will calculate the difference in minutes between each pair of timestamps in the table, as long as the second timestamp is greater than the first one (so we don't get negative values). The WHERE clause ensures that we only join pairs of rows with different IDs.

1. Use a self join with a subquery:

If your data has some other form of ordering (not based on a timestamp), you can use a self join with a subquery to join each row with the row that comes right after it in the ordering. Here's an example of how to do that:

SELECT t1.value, t2.value, t2.id - t1.id AS id_diff
FROM my_table t1
JOIN my_table t2 ON t2.id = (
   SELECT MIN(id) FROM my_table WHERE id > t1.id
)
ORDER BY t1.id;

This will join each row with the row that has the next highest ID, and then calculate the difference in IDs between those two rows.

1. Use variables to keep track of the previous row:

If you don't want to use a self join or a subquery, you can use user-defined variables to keep track of the previous row as you iterate through the table. Here's an example of how to do that:

SELECT id, value, TIMESTAMPDIFF(SECOND, prev_timestamp, timestamp) AS elapsed_secs
FROM (
   SELECT id, value, timestamp, @prev_timestamp AS prev_timestamp, @prev_timestamp := timestamp
   FROM my_table
   CROSS JOIN (SELECT @prev_timestamp := NULL) vars
   ORDER BY timestamp
) t

This will iterate through the table in timestamp order, and use user-defined variables to keep track of the previous timestamp as it goes. The outer SELECT statement then calculates the difference in seconds between each pair of timestamps. Note that this approach assumes that the timestamps are unique; if there are duplicates, you'll need to add some extra logic to handle that case.