MySQL: How many minutes ago was DB updated?

I need to keep a field in a data-base and update it with a time somehow, then later I need to check that time to see if it was over 30 minutes ago or not, and if not, how minutes left until 30?

I am going to be doing this with PHP+MySql can anyone tell me the simplest way to do this?

Thanks!!

Solution

Let's assume you want to know how long ago the last update/insert in the table occurred.
You can set up a table with a timestamp field with an on update clause

CREATE TABLE foo (
  id int auto_increment,
  ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  primary key(id),
  key(ts)
)

and then query the record with the largest value in ts

SELECT
  TIMEDIFF(Now()-Interval 30 Minute, ts)
FROM 
  foo
ORDER BY
  ts DESC
LIMIT
  1

edit: This also works if you want to get all records that have been inserted/modified within e.g. the last 12 hours.

SELECT
  TIMEDIFF(Now()-Interval 30 Minute, ts)
FROM 
  foo
WHERE
  ts > Now()-Interval 12 hour
ORDER BY
  ts DESC

edit2: there's also an off chance you might be interested in http://dev.mysql.com/doc/refman/5.1/en/show-table-status.html:

SHOW TABLE STATUS returns the following fields:
...
Update_time
When the data file was last updated. For some storage engines, this value is NULL. For example, InnoDB stores multiple tables in its tablespace and the data file timestamp does not apply. For MyISAM, the data file timestamp is used; however, on Windows the timestamp is not updated by updates so the value is inaccurate.