Mysql Timestamp difference results - mysql

Question

I have a table with a TIMESTAMP field (lastHonored).
I ran this query:
SELECT NOW(), lastHonored,
TIMESTAMPDIFF(SECOND, lastHonored, NOW()), NOW()-lastHonored
FROM db.table
I get the result:
NOW() | lastHonored | DIFF | SUBTRACT
2014-10-27 14:07:22 | 2014-10-26 19:49:51 | 65851 | 945771
Where DIFF is the result of the TIMESTAMPDIFF function, and SUBTRACT is the result of the NOW()-lastHonored expresssion.
DIFF looks right, but can anyone tell me what NOW()-lastHonored calculates? It is not the right order of magnitude, and I'm stumped.

Answer 1

One would think that NOW() returns a datetime or similar type. But no. For some historical reason, NOW() returns either a number or a string. To quote the documentation:
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS'
or YYYYMMDDHHMMSS format, depending on whether the function is used in
a string or numeric context. The value is expressed in the current
time zone.
That means that NOW() gets converted to a value based on its context. The - suggests a numeric context, so NOW() is a number whose digits are YYYYMMDDHHMMSS. My guess is that lastHonored gets similarly converted, so the result is the difference between two numbers.

Answer 2

You can see why by running:
SELECT CAST(NOW() AS UNSIGNED), CAST('2014-10-26 19:49:51' AS UNSIGNED);
By doing simple subtraction, MySQL is turning both values into numbers. NOW() the DATETIME becomes 20141027141923, but 2014-10-26 19:49:51 the STRING becomes 2014.
If you first cast the date to a DATETIME it gives you results more along the lines of what you expect:
SELECT CAST(NOW() AS UNSIGNED), CAST(CAST('2014-10-26 19:49:51' AS DATETIME) AS UNSIGNED);

Answer 3

You can't subtract dates like you are with NOW()-lastHonored. Dates/datetimes are not directly "subtractable":
MariaDB [test]> select '2014-10-27 08:18:00' - '2014-10-27 08:17:00';
+-----------------------------------------------+
| '2014-10-27 08:18:00' - '2014-10-27 08:17:00' |
+-----------------------------------------------+
| 0 |
+-----------------------------------------------+
1 row in set, 2 warnings (0.00 sec)
MariaDB [test]> show warnings;
+---------+------+---------------------------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: '2014-10-27 08:18:00' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '2014-10-27 08:17:00' |
+---------+------+---------------------------------------------------------+
2 rows in set (0.00 sec)
Note the warnings. You'll get results, but they almost always be totally useless/incorrect results, because MySQL is casting the date values as doubles.

Answer 4

They do different things, substraction does not take into account that you are dealing with times.
If you do:
SELECT NOW(), lastHonored,
TIMESTAMPDIFF(SECOND, lastHonored, NOW()) THE_DIFF, NOW()-lastHonored THE_SUBSTRACTION,
NOW()+0 NOW_NUMBER_REPRESNTATION,
lastHonored+0 lastHonored_NUMBER_REPRESENTATION
FROM db.table
You will see the numeric difference is represented with the substraction and the time differente with TIMESTAMPDIFF.