Question

I seem to have a problem with this SQL query:

SELECT * FROM appts  WHERE timeStart >='$timeStart'  AND timeEnd <='$timeEnd'  AND dayappt='$boatdate' 

The time is formatted as military time. The logistics is that a boat rental can be reserved at 7am til 1pm or 9am til 1pm or 9am til 5pm. If there is an appt within that range, it should return appts but it has proven to be inconsistent. If I pick 9am til 1pm, it will ignore appts that started with 7am even though it overlaps 9am-1pm. If I pick 9 to 5, it will return nothing even though it should with the 7am to 1pm. How do I make a SQL statement that includes the whole range from timeStart to timeEnd including those that overlap?

Answer 1

The correct check would look like this:

SELECT * FROM appts  WHERE timeStart <='$timeEnd'  AND timeEnd >='$timeStart'  AND dayappt='$boatdate' 

Other good explanations have been given but I’ll go ahead and update it with an alternative explanation of how I visualize this myself. Most people are looking for each possible overlap, considering two time periods, they are trying to think of each combination of start and end that can make an appointment overlap. I think about it as when do two time periods not overlap which for some reason is easier for me.

Say the time period I am checking for is today, I want to find any time period that does not overlap today. There are really only two scenarios for that, either the time period starts after today (PeriodStart > EndOfToday) or the time period ends before today (PeriodEnd < StartOfToday).

Given that we havea simple test for not overlapping:
(PeriodStart > EndOfToday) OR (PeriodEnd < StartOfToday)

A quick flip around and you have a simple test for overlap:
(PeriodStart <= EndOfToday) AND (PeriodEnd >= StartOfToday)

-Shane