Question

I wasn’t sure what to put into the title for this…please read on…

I have a table of events with 3 days per event

eventID dayID
1       1
1       2
1       3
2       1
2       2
2       3

I also have a number of cars available for the event

carID regNum
1     fg12 bfr
2     gh34 vbn
3     kl56 thg
4     lm32 tfx

I would like to have certain cars available for certain events, for example carIDs 1,2 and 3 might be available for eventID 1 but only carID 4 is available for event 2.

I am unsure how to join these tables together.

I am looking at another table which defines which cars are available to which event for example

eventID carID
1       1
1       2
1       3
2       4

but I can’t work out how to join these 3 tables together to get the car list for event1 for example.

Any pointers gratefully received!

Answer 1

Junction tables are the classic solution for modeling a many-to-many relationship in an RDBMS.

You do not need to join three tables to get cars for event1 – two tables is enough:

select *
from Cars c
join CarEvent e on e.CarId = c.CarId
where e.EventId = 1

If you need the event name, you need to join the event table as well:

select *
from Cars c
join CarEvent ce on ce.CarId = c.CarId
join Event e on e.EventId = ce.EventId
where e.EventName = 'Event1'