计算MySQL表中两个时间字段之间的时差

Question

I want to calculate difference between two time fields in a view (mysql). I'm using this query:

$result = mysql_query("CREATE OR REPLACE VIEW arbtim AS SELECT week, name, start, stop, arton,
      ( arton ) - ( start ) AS t18 FROM arbtid;");

The field "arton" is always "18:00:00". If start = "10:00:00" it returns "80000". If start = "09:45:00" it returns "85500". I want it to be "8" and 8,25

Both fields are in time format. The created field t18 is in the double format - why?

Answer 1

Reading the documentation ( http://dev.mysql.com/doc/refman/5.7/en/date-and-time-type-conversion.html ), unfortunately 85500 is the documented behaviour. I know what you are thinking... that sucks :(

The problem is that TIME is converted to 10000 * hours + 100 * minutes + seconds.

Since you want hours, I suggest you do it explicitly:

(3600*HOUR(arton)+60*MINUTE(arton)+SECOND(arton)
-3600*HOUR(start)-60*MINUTE(start)-SECOND(start))/3600 AS t18

which (thanks to GolezTrol), with the TIME_TO_SEC function, can be more simply written as:

(TIME_TO_SEC(arton)-TIME_TO_SEC(start))/3600 AS t18

---

But I want to go deeper with this answer, because this strange TIME algebra bothers me a lot. I've never seen something like this in any other RDBMS or language. Mind you, I haven't seen them all, but quite a few.

What bothers me is that this TIME algebra is a trap, especially for the inexperienced.

There are just too many things that can go wrong with this. Here's a partial list:

• 100 is 00:01:00, but 100-1 is not 00:00:59, but an invalid time
• 100 is 00:01:00, but 100*100 is not 01:40:00, but 01:00:00
• 100 is 00:01:00, but 100/2 is not 00:00:30, but 00:00:50

... and many others I might add as I think of them.

Do you see where I am getting? Do not operate on time columns... not in this warped and incomplete space MySQL provides us. In this space most operations will not work as we would expect them to.

The only reasonable solution here is to get out... convert those times to something else we can sum, subtract, divide and multiply as we would expect. TIME_TO_SEC is a good simple choice, but there are others. And do convert TIME values before doing anything else with them.

Answer 2

There are a couple of correct ways of doing it:

Using timestamps - in this case, the result will be in seconds:

SELECT week, name, start, stop, arton,  UNIX_TIMESTAMP( arton ) - UNIX_TIMESTAMP( start ) AS t18 FROM arbtid;

Then, to get hours, divide the number of seconds by 3,600.

Or using TIMEDIFF - in this case, the result will be a time value like "-00:00:00.000001":

SELECT week, name, start, stop, arton,  TIMEDIFF( arton, start ) AS t18 FROM arbtid;

I think the former is probably more suitable for your application.

Answer 3

You can use TIME_TO_SEC to get the number of seconds in the time span. Then, you can use that to calculate the hours, by simply dividing it by 3600.

SELECT TIME_TO_SEC(arton - start) / 3600 AS HoursBetween FROM arbtid