How to add random interval to timestamp in MySQL? - mysql

Question

We got the following table mytable:
+----+------------+------------+
| id | created | expired |
+----+------------+------------+
| 1 | 1496476314 | NULL |
| 6 | 1496477511 | NULL |
| 7 | 1496477518 | NULL |
| 12 | 1496477534 | NULL |
| 13 | 1496477536 | NULL |
| 15 | 1496477541 | NULL |
| 21 | 1496477548 | NULL |
| 22 | 1496477550 | NULL |
| 26 | 1496477565 | NULL |
| 28 | 1496477566 | NULL |
| 29 | 1496477583 | NULL |
+----+------------+------------+
We'd like to do the following:
set expired = created + random(15 - 30 minutes) as unix_timestamp where expired is null;
I currently have no idea to done it.
If u just can give me some ideas it would save my day.
I tried to convert the created timestamp to date_time and add to that date_time the wanted 15 - 30 minutes and finally convert the new_date_time back to unix_timestamp, but there should be an easier way.

Answer 1

If you want to add a random number of minutes between, say, 14 and 33, you can do it like this:
SET expired = DATE_ADD(created, INTERVAL 14 + RAND()*(33-14) MINUTE);
If you want to have seconds granularity, you need to add SECOND-typed intervals:
SET expired = DATE_ADD(created, INTERVAL 14*60 + RAND()*(33-14)*60 SECOND);
This would saves one datetime conversion if you had a DATETIME for the expired column, which makes it slightly easier to expire records (WHERE expired < NOW()). If you have an integer holding a Unix timestamp, then Darshan's answer is definitely the way to go, and you'd do well to calculate the Unix timestamp in your app and then plug it in the query:
WHERE expired <= 123456789
Having an index on that column would make expirations go blazingly fast. I think it might be even faster than the datetime method, but it's just a sensation, I haven't actually checked.

Answer 2

unix_timestamp is number of seconds elapsed since 1st January 1970. Now, if you want to add 15 to 30 minutes then the equivalent seconds would be 900 to 1800. Here's what you can do:
set expired = created + ROUND((RAND() * (900))+900) where expired is null;
This is how it works:
RAND() will generate a random number between 0 and 1
By using RAND() * (maximum - minimum)) + minimum we make sure we generate a number between 900 and 1800.
ROUND then rounds that number to nearest int.