[database-design] Calendar Recurring/Repeating Events - Best Storage Method

While the currently accepted answer was a huge help to me, I wanted to share some useful modifications that simplify the queries and also increase performance.

"Simple" Repeat Events

To handle events which recur at regular intervals, such as:

Repeat every other day 

or

Repeat every week on Tuesday 

You should create two tables, one called events like this:

ID    NAME
1     Sample Event
2     Another Event

And a table called events_meta like this:

ID    event_id      repeat_start       repeat_interval
1     1             1369008000         604800            -- Repeats every Monday after May 20th 2013
1     1             1369008000         604800            -- Also repeats every Friday after May 20th 2013

With repeat_start being a unix timestamp date with no time (1369008000 corresponds to May 20th 2013) , and repeat_interval an amount in seconds between intervals (604800 is 7 days).

By looping over each day in the calendar you can get repeat events using this simple query:

SELECT EV.*
FROM `events` EV
RIGHT JOIN `events_meta` EM1 ON EM1.`event_id` = EV.`id`
WHERE  (( 1299736800 - repeat_start) % repeat_interval = 0 )

Just substitute in the unix-timestamp (1299736800) for each date in your calendar.

Note the use of the modulo (% sign). This symbol is like regular division, but returns the ''remainder'' instead of the quotient, and as such is 0 whenever the current date is an exact multiple of the repeat_interval from the repeat_start.

Performance Comparison

This is significantly faster than the previously suggested "meta_keys"-based answer, which was as follows:

SELECT EV.*
FROM `events` EV
RIGHT JOIN `events_meta` EM1 ON EM1.`event_id` = EV.`id`
RIGHT JOIN `events_meta` EM2 ON EM2.`meta_key` = CONCAT( 'repeat_interval_', EM1.`id` )
WHERE EM1.meta_key = 'repeat_start'
    AND (
        ( CASE ( 1299132000 - EM1.`meta_value` )
            WHEN 0
              THEN 1
            ELSE ( 1299132000 - EM1.`meta_value` )
          END
        ) / EM2.`meta_value`
    ) = 1

If you run EXPLAIN this query, you'll note that it required the use of a join buffer:

+----+-------------+-------+--------+---------------+---------+---------+------------------+------+--------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref              | rows | Extra                          |
+----+-------------+-------+--------+---------------+---------+---------+------------------+------+--------------------------------+
|  1 | SIMPLE      | EM1   | ALL    | NULL          | NULL    | NULL    | NULL             |    2 | Using where                    |
|  1 | SIMPLE      | EV    | eq_ref | PRIMARY       | PRIMARY | 4       | bcs.EM1.event_id |    1 |                                |
|  1 | SIMPLE      | EM2   | ALL    | NULL          | NULL    | NULL    | NULL             |    2 | Using where; Using join buffer |
+----+-------------+-------+--------+---------------+---------+---------+------------------+------+--------------------------------+

The solution with 1 join above requires no such buffer.

"Complex" Patterns

You can add support for more complex types to support these types of repeat rules:

Event A repeats every month on the 3rd of the month starting on March 3, 2011

or

Event A repeats second Friday of the month starting on March 11, 2011

Your events table can look exactly the same:

ID    NAME
1     Sample Event
2     Another Event

Then to add support for these complex rules add columns to events_meta like so:

ID    event_id      repeat_start       repeat_interval    repeat_year    repeat_month    repeat_day    repeat_week    repeat_weekday
1     1             1369008000         604800             NULL           NULL            NULL          NULL           NULL             -- Repeats every Monday after May 20, 2013
1     1             1368144000         604800             NULL           NULL            NULL          NULL           NULL             -- Repeats every Friday after May 10, 2013
2     2             1369008000         NULL               2013           *               *             2              5                -- Repeats on Friday of the 2nd week in every month    

Note that you simply need to either specify a repeat_interval or a set of repeat_year, repeat_month, repeat_day, repeat_week, and repeat_weekday data.

This makes selection of both types simultaneously very simple. Just loop through each day and fill in the correct values, (1370563200 for June 7th 2013, and then the year, month, day, week number and weekday as follows):

SELECT EV.*
FROM `events` EV
RIGHT JOIN `events_meta` EM1 ON EM1.`event_id` = EV.`id`
WHERE  (( 1370563200 - repeat_start) % repeat_interval = 0 )
  OR ( 
    (repeat_year = 2013 OR repeat_year = '*' )
    AND
    (repeat_month = 6 OR repeat_month = '*' )
    AND
    (repeat_day = 7 OR repeat_day = '*' )
    AND
    (repeat_week = 2 OR repeat_week = '*' )
    AND
    (repeat_weekday = 5 OR repeat_weekday = '*' )
    AND repeat_start <= 1370563200
  )

This returns all events that repeat on the Friday of the 2nd week, as well as any events that repeat every Friday, so it returns both event ID 1 and 2:

ID    NAME
1     Sample Event
2     Another Event

*Sidenote in the above SQL I used PHP Date's default weekday indexes, so "5" for Friday

Hope this helps others as much as the original answer helped me!