difference of time MYSQL PHP - php

Question

How can I get an update another field (whole column) the difference of time in (TIME 00:00:00) format in my MYSQL phpMyAdmin database
<?php
$con=mysqli_connect("***","****","****","******");
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
mysqli_query($con,"UPDATE members SET time_on = current_time-logon_time");
?>
I keep getting 0:00:00 and no results when I do this in PHP. The other times our fine, just trying to get the difference and update all fields in the time_on column.

Answer 1

You should use timediff()
mysqli_query($con,"UPDATE members SET time_on = timediff(current_time,logon_time)");

Answer 2

You can use TIMESTAMPDIFF() to get the difference between two dates and/or datetimes.
Try this:
UPDATE members
SET time_on = TIMESTAMPDIFF(SECOND, logon_time, current_time)
This will set the difference in seconds.

Answer 3

Just to expand on why the other answers are right: You're doing MATH on times, which doesn't work as you expect:
mysql> create table foo (x time, y time);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into foo (x, y) values ('14:15:16', '09:08:07');
Query OK, 1 row affected (0.00 sec)
mysql> select * from foo;
+----------+----------+
| x | y |
+----------+----------+
| 14:15:16 | 09:08:07 |
+----------+----------+
1 row in set (0.00 sec)
mysql> select x - y from foo;
+-------+
| x - y |
+-------+
| 50709 |
+-------+
1 row in set (0.00 sec)
Note how the result is NOT a time value, it's an integer. It happens to be a direct time->string->numeric conversion of 05:07:09, which is the literal as-expected time difference of the two values. But since you're doing MATH, i.e. subtraction, MySQL is assuming you want a numeric answer, and giving it to you.
That's why there's timediff() and similar date/time functions, which return a real date/datetime value, not this wonky/useless integer.