Mysql subtraction operator on timestamps [duplicate] - mysql

Question

This question already has answers here:
What is the behavior for the minus operator between two datetimes in MySQL?
(2 answers)
Closed 1 year ago.
I wrote a query like
select endtime - begintime ....
and it looked like the difference in seconds. But it turns out that it is very odd number (both columns of type timestamp, no timezones mentioned).
select timestampdiff(seconds, begintime, endtime)
works.
But I am more than a little curious as to what the subtraction operator does! I could not find any documentation. It is certainly a booby trap for new users.
(And nobody really understand timezones. There is what is stored, vs what is displayed in different time zones, which drivers etc. muck with it, and lots and lots of false information and confusion. I don't know what With Timezone really means, but I only use the one timezone of the server, although my browser is in a different timezone so phpadmin might be lying to me.)

Answer 1

When used as a number, a timestamp like '2021-01-02 03:04:05' will be treated as 20210102030405. You can see this with e.g. select timestamp('2021-01-02 03:04:05')-0;. Subtracting two such "numbers" isn't going to be meaningful, except that the sign of the result will tell you which time was later.
This doesn't apply if you use the special INTERVAL syntax to adjust a timestamp by an interval, e.g. select '2021-01-02 03:04:05' - INTERVAL 1 WEEK;.

Answer 2

Here's a demo:
mysql> create table mytable (endtime datetime, begintime datetime);
Query OK, 0 rows affected (0.05 sec)
mysql> insert into mytable values (now(), '2021-05-01');
mysql> select endtime - begintime from mytable;
+---------------------+
| endtime - begintime |
+---------------------+
| 6011403 |
+---------------------+
What's up with this weird value? Well, when you put datetime values into an integer arithmetic expression, they values are converted to integers, but not in units of seconds. You can also force these values to be integers this way:
mysql> select endtime+0 as e, begintime+0 as b from mytable;
+----------------+----------------+
| e | b |
+----------------+----------------+
| 20210507011403 | 20210501000000 |
+----------------+----------------+
Here we see that the values are integers, but they are based on converting the datetime values to YYYYMMDDHHMMSS format.
Guess what the difference is?
mysql> select e-b from (select endtime+0 as e, begintime+0 as b from mytable) as t;
+---------+
| e-b |
+---------+
| 6011403 |
+---------+
But this is not the actual time difference, because there are not 100 minutes in an hour, 100 hours in a day, etc.
mysql> select timestampdiff(second, begintime, endtime) as timestampdiff from mytable;
+---------------+
| timestampdiff |
+---------------+
| 522843 |
+---------------+