datetime - Why do I get NULL results with the MySQL function TIME_TO_SEC? -


i have mysql table 2 datetime columns. want create 1 new generated column contains time difference of 2 datetime columns. using mysql function time_to_sec convert difference seconds.

strangely of rows return null, , unable understand why. here first 25 rows of table..

 class_session_id starts_on           ends_on              session_time_computed                 8 2014-12-24 03:35:47 2014-12-24 05:07:33                    na                 9 2014-12-24 05:50:37 2014-12-24 07:10:07                    na                10 2014-12-24 07:18:51 2014-12-24 08:52:27                    na                11 2014-12-19 09:00:41 2014-12-19 10:35:54                  5713                12 2014-12-19 10:45:29 2014-12-19 11:55:48                  4219                13 2014-12-24 12:00:00 2014-12-24 14:00:00                  7200                14 2014-12-24 14:32:10 2014-12-24 14:47:18                   908                15 2014-12-24 14:48:13 2014-12-25 08:29:51                    na                16 2014-12-15 04:06:13 2014-12-15 04:20:41                   868                17 2014-12-15 07:09:27 2014-12-15 08:20:40                  4273                18 2014-12-15 06:42:28 2014-12-15 06:51:43                   555                28 2014-11-25 11:18:00 2014-11-25 09:30:00                    na                29 2014-11-27 10:18:00 2014-11-27 11:30:00                  4320                30 2014-11-26 08:18:00 2014-11-26 09:30:00                  4320                34 2015-01-08 10:41:52 2015-01-08 11:00:00                  3528                37 2014-12-02 12:44:36 2014-12-02 14:44:00                    na                49 2014-11-02 12:44:00 2014-11-02 14:44:00                  7200                54 2014-12-07 07:05:35 2014-12-07 07:50:00                    na                55 2014-12-07 10:00:00 2014-12-07 11:00:00                  3600                56 2014-11-12 12:57:00 2014-11-12 17:00:00                 16980                82 2014-12-05 14:29:18 2014-12-05 16:00:00                    na                83 2014-12-05 16:29:18 2014-12-05 17:00:00                    na                84 2014-12-05 17:49:17 2014-12-05 18:25:00                    na                85 2014-12-06 15:00:17 2014-12-06 16:00:00                    na                87 2014-12-06 16:25:00 2014-12-06 16:59:00                  2040`

i use following script generate last column.

 alter table class_sessions_archive         modify column session_time_computed int(11) (time_to_sec(ends_on - starts_on));

why null results many rows? here structure of table :

           field     type null key default                   class_session_id  int(11)   no pri    <na>    auto_increment               class_id  int(11)   no mul    <na>                                  room_id  int(11)   no mul    <na>                               teacher_id  int(11)   no mul    <na>                                starts_on datetime   no        <na>                                  ends_on datetime   no        <na>                            session_state  int(11)   no mul    <na>                            stud_attended int(100)  yes        <na>                    total_stud_registered  int(11)   no           0                             session_time  int(11)   no           0                                   gi_num  int(11)   no           0                                   gi_den  int(11)   no           0                                       pi  int(11)   no           0                    session_time_computed  int(11)  yes        <na> virtual generated

now think it, convert time seconds before subtracting:

alter table class_sessions_archive         modify column session_time_computed int(11)             (to_seconds(ends_on) - to_seconds(starts_on));

otherwise, limited range of values time data type -- why getting null.


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