跨越三个表的SQL查询

Question

Bear with me please because my SQL is not that good.

I want to write a query that takes into consideration information from 3 different tables.

Let me explain the situation: When a customer books a room, they have the preference of several rooms and the rooms can be booked on individual days (one unique booking has to be made per day) at different times.

I now want to write a query that will check if a room is available on a certain time of the day. These are the tables I have on my database

room

This table stores information about each room

Fields:

id (primary key)
capacity    
img 
notes   
building_id 
roomstructure_id    
lecturestyle_id

request

This table stores message about each request made

Fields

id (primary key)
day_id
period_id
preference_id (foreign key from preference.id)

This table stores information about each request made.

preference

This table stores information about all the room preferences.

id (PK)      
request_id (foreign key from request.id)      
room_id (foreign key from room.id)

I hope what I am trying to do has become apparent.

If for example I am checking whether room C233 is booked on Monday, 09:00 - I would need to look up the room information in the room table and then use the room_id in the preference table to look up the request_id .

Once I have the request_id which has been linked to a specific room (in the room preference table) I can see whether day_id=1 and period_id=1 in the request table. If day_id=1 and period_id=1 then it means that the room is booked at Monday, 09:00.

This means that room is not available so it isn't counted but if this row could not be found then it would be counted.

This is the SQL I've written so far - its very simple but it doesn't do everything I want it to do:

SELECT COUNT(*) totalCount FROM ts_room

In addition to this, I have been offered advice on another forum - this code here. But it doesn't work at all. I don't think it meets my criteria:

SELECT  COUNT(*) totalCount
FROM    room a
        INNER JOIN preference b
            ON a.id = b.room_id
        INNER JOIN ts_request c
            ON b.request_id = c.preference_id
WHERE   c.day_id = 1 AND c.period_id = 1

Thanks in advance!

Answer 1

OK, that code you posted is close to correct, but has a couple of problems. First, there's an inconsistency in your name for the request table. In your description you just call it 'request', but in the code it's 'ts_request'. Might just be a typo in your question though.

The other issue is that the keys in your second INNER JOIN (between preference and request) are mixed up. You have b.request_id = c.preference_id. IE: you're trying to match the two foreign keys up with each other, rather than with the primary keys that they represent. You only need one primary/foreign key pair to do the join. So this should work (not tested):

SELECT  COUNT(*) totalCount
FROM    room a
        INNER JOIN preference b
            ON a.id = b.room_id
        INNER JOIN request c
            ON b.request_id = c.id
WHERE   c.day_id = 1 AND c.period_id = 1

(I'm not a fan of using totally non-descriptive identifiers a, b, and c, but it's valid code. Personally I would at least use rm, rq, pf, or something identifiable like that. Or even just stick with the table names since they're fairly short anyway.)

One thing to note is that having foreign keys in both directions from the request and preferences tables means there is exactly one preference per request. (Or possibly zero if the preference_id field in the request table can be left blank/null.) If you intended it to be possible for there to be multiple preferences submitted per request, that preference_id foreign key column should be removed, since it's not a unique value. It's also not required for the above query, although if indeed there is indeed a limitation of one preference per request, it's obviously possible you've included it for use somewhere else.

Edit: As mentioned in the comments on your question, this query also doesn't look for a specific room, but rather returns the total number of rooms reserved for day 1, period 1. If you want a specific room, you would also have to add "AND a.id = " to your WHERE clause.