sql获取未在特定日期和时间预订的所有可用人员

Question

I'm struggling with an SQL query. I am building a booking system for a ski resort and in my database I have instructors and sessions. A session can have an instructor, and it has a date and startTime and endTime. In order to add a session, I want to get all available instructors for a chosen time and date. In other words, all instructors who don't have a booking on that date and at that time. Table example:

eg instructors: i1, i2, i3, i4, i5, i6, i7, i8

sessions:

Instructor | date | start | end |

**i1**          **2017-05-03**        **14:30:00**      **15:30:00**
**i2**          **2017-05-03**        **14:30:00**      **15:30:00**
**i3**          **2017-10-03**        **10:30:00**      **11:30:00**
**i4**          **2017-05-03**        **10:30:00**      **11:30:00**
**i1**          **2017-11-03**        **14:30:00**      **15:30:00**

Then for input date='2017-05-03' and start='14:30' and end='15'30' i want to get

i3,i4,i5,i6,i7,i8

Figured out that I need to left join session to instructors, group by instructor id and then eliminate those ids that have a field in the group with the selected inputs. However, for the GROUP BY clause, i have to use an aggregate function and i don't know which one could apply here.

Answer 1

using not exists()

select *
from instructors i
where not exists (
  select 1
  from sessions s
  where s.instructor = i.id
    and s.date  = '2017-05-03'
    and s.start = '14:30'
    and s.end   = '15:30'
    )

Answer 2

So I tried this query and apparently it works(at least for my test case)

Can anybody take a look and tell me if it looks correct?

select * 
from instructor
where id in
(select id
from instructor 
group by id
having id not in 
(select distinct(instructorid) from Session
where date='2017-03-19' and starttime<='15:30:00' and endtime>='14:30:00') )

Answer 3

SirWinning's self-answer looks like it should work, but my version below removes some parts which weren't required.

select * 
from instructor
where id not in 
  (select instructorid
   from Session
   where date='2017-03-19' and starttime<='15:30:00' and endtime>='14:30:00')

This code will find any instructors who aren't booked for a session which overlaps the 14:30-15:30 window on the relevant date.

If that's what's wanted, then you're good to go. Of course it doesn't follow that the instructor is "really available". There could be other things which affect their availabilty (working hours, annual leave etc), so you'll need to ensure that there are things in place to handle such things.

Note also, that this code will prevent an instructor appearing available for "back to back" bookings. If you want to allow a booking to start at 14:30 when another one ends at that time, you'll need to change the <= and >= to < and > .