MySQL Getting the return of a substraction

Question

We are trying to get the length of a physical exercise by using the timestamps on our sensor data.

We currently have the following query:

SELECT UNIX_TIMESTAMP(
    SELECT HAAS2.trainingsdata.timestamp
    FROM HAAS2.trainingdata
    WHERE HAAS2.trainingsdata.training_id= 1
    ORDER BY timestamp DESC LIMIT 1)
- UNIX_TIMESTAMP(
    SELECT HAAS2.trainingsdata.timestamp
    FROM HAAS2.trainingdata
    WHERE HAAS2.trainingsdata.training_id= 1
    ORDER BY timestamp ASC LIMIT 1)
AS output

(enters added for readability)

When testing this query in phpMyAdmin we get the following error:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT HAAS2.trainingsdata.timestamp FROM HAAS2.trainingdata WHERE HAAS2.trainin' at line 1

We've tried different ways to write down the query all resulting in the same error. We don't understand where the syntax error lies.

Answer 1

SELECT max(UNIX_TIMESTAMP(timestamp)) -
       min(UNIX_TIMESTAMP(timestamp)) AS output
FROM HAAS2.trainingdata
WHERE training_id = 1