Flipflops.org

Quite often you end up in situations where you are required to check and see if one time period overlaps another time period. Probably the most common situation this occurs in is when you are building a booking system - be it for tables or cars or rooms.

You have a series of entries in a database with a start (date)time and an end (date)time and before adding a new record you need to check that it doesn’t overlap with another booking.

The solution is fairly simple but I always end up making a quick diagram on a piece of paper to check.

As you can see two time periods can either be sequential (i.e. there is no overlap at all) or they can overlap in one of four ways. Put this into a diagram and the solution becomes pretty obvious.

There is an overlap if end_time_1 > start_time_2 AND start_time_1 < end_time_2

An example SQL query would be something like the one below but make sure your datetime formats are correct - in MySQL the default format is YYYY-MM-DD hh:mm:ss

 
SELECT * FROM bookings WHERE room_id = '" . $room_id . "' AND ((date_end > '" . $start_date . "') AND (date_start < '" . $end_date . "')) ";

The above query will pull back the records where the times overlap.

Like many (most?) developers when I work with a database I tend to manage them using another programme - usually either phpMyAdmin or Navicat. However there are times when you have to get basic again and dig down to the Command Line (usually because of an access issue like a firewall setting you can’t change or because you need to import a file too big for phpMyAdmin). The trouble is if you only use these commands once in a blue moon you soon forget them.

Dump (export your database)

1. Login to the shell or Command Prompt, navigate to where you would like your file saved.
2. 
mysqldump --user=your_username --password=your_password your_database > your_dump_file.sql


Where:
your_username is a mysql user with the correct rights to the database you want to dump.
your_password is the password for your_username.
your_database is the name of your database.
your_dump_file.sql is the name of the file that will be created in the export.

Import

1. Create the database that you want to import into, but leave it empty or if you are using an existing database remove the tables (make sure you have a backup first).
2. Login to the shell or Command Prompt
3. 
mysql --user=your_username --password=your_password your_database < your_file_to_import.sql