高级SQL选择查询/动态列

Question

I'm leaning MySQL but struggling with more advanced queries, so I hope someone could shed some light

I have the following database (I hope it's properly formated)

table_visitors
visitor_id | visitor_name
   1       |       Joe
   2       |       Bob

table_rooms
room_id | room_name
    1   |  room 1
    2   |  room 2 
    3   |  room 3
    4   |  room 4

table_roomsvisitors (indicates which visitors can access which rooms)
visitor_id | room_id
    1      |    1
    1      |    2
    1      |    3
    2      |    1
    2      |    4 

I'd like to list (for a specific visitor) every item in table_rooms as well if the selected visitor is allowed to access each room

The expected result should be something like that:

query for Joe:
room_name | access
 room 1   | true
 room 2   | true
 room 3   | true
 room 4   | false

query for Bob:
room_name | access
 room 1   | true
 room 2   | false
 room 3   | false
 room 4   | true

Answer 1

You could use a left join and case statement to check if user has access to room

select r.room_name, 
       case when rv.visitor_id is not null then true else false end access
from table_rooms r
left join table_roomsvisitors rv on r.room_id = rv.room_id
and rv.visitor_id = 1

demo

left join will produce null where there is no association found for room in table_roomsvisitors based on additional join condition as rv.visitor_id = 1 which will join only rows from table_roomsvisitors where visitor_id is one and for other rows it will return null, thus in select part you could check for not null rows and return true, false for null rows

Answer 2

You cam use CROSS JOIN :

select v.visitor_name, r.room_name, 
       (case when rv.visitor_id is null then 'false' else 'true' end) as access
from table_visitors v cross join 
     table_rooms r left join
     table_roomsvisitors  rv 
     on rv.visitor_id = v.visitor_id and rv.room_id = r.room_id
where v.visitor_name = 'Joe';

Answer 3

joined the tables and then used a case for true and false. Then the id is based on who since you said a query for bob and one for joe

For Joe:

select tr.room_name, case when rv.visitor_id is not null then 'true' else 'false' end access from table_rooms tr
left join table_roomvisitors rv on tr.room_id = rv.room_id
where rv.visitor_id = 1

For Bob:

select tr.room_name, case when rv.visitor_id is not null then 'true' else 'false' end access from table_rooms tr
left join table_roomvisitors rv on tr.room_id = rv.room_id
where rv.visitor_id = 2