How to store SQL Query result in table column - mysql

Question

I'm aware of the INSERT INTO table_name QUERY; however, I'm unsure how to go about achieving the desired result in this case.
Here's a slightly contrived example to explain what I'm looking for, but I'm afraid I cannot put it more succiently.
I have two tables in a database designed for a hotel.
BOOKING and CUSTOMER_BOOKING
Where BOOKING contains PK_room_number, room_type, etc. and CUSTOMER_BOOKING contains FK_room_number, FK_cusomer_id
CUSTOMER_BOOKING is a linking table (many customers can make many bookings, and many bookings can consist of many customers).
Ultimately, in the application back-end I want to be able to list all rooms that have less than 3 customers associated with them. I could execute this a separate query and save the result in the server-side scripting.
However, a more elegant solution (from my point of view) is to store this within the BOOKING table itself. That is to add a column no_of_bookings that counts the number of times the current PK_room_number appears as the foreign key FK_room_number within the CUSTOMER_BOOKING table. And why do this instead? Because it would be impossible for me to write a single complicated query which will both include the information from all ROOMS, among other tables, and also count the occurrences of bookings, without excluding ROOMS that don't have any bookings. A very bad thing for a hotel website attempting to show free rooms!
So it would look like this
BOOKING: PK_room_number (104B) room_type (double) room_price (high), no_of_bookings (3)
BOOKING: PK_room_number (108C) room_type (single) room_price (low), no_of_bookings (1)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (4312)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (6372)
CUSTOMER_BOOKING: FK_room_number (104B) FK_customer_id (1112)
CUSTOMER_BOOKING: FK_room_number (108C) FK_customer_id (9181)
How would I go about creating this?

Answer 1

Because it would be impossible for me to write a single complicated
query which will both include the information from all ROOMS, among
other tables, and also count the occurrences of bookings, without
excluding ROOMS that don't have any bookings.
I wouldn't say it's impossible and unless you're running into performance issues, it's easier to implement than adding a new summary column:
select b.*, count(cb.room_number)
from bookings b
left join customer_booking cb on b.room_number = cb.room_number
group by b.room_number
Depending on your query may need to use a derived table containing the booking counts for each room instead instead
select b.*, coalesce(t1.number_of_bookings,0) number_of_bookings
from bookings b
left join (
select room_number, count(*) number_of_bookings
from customer_booking
group by room_number
) t1 on t1.room_number = b.room_number
You have to left join the derived table and select coalesce(t1.number_of_bookings,0) in case a room does not have any entries in the derived table (i.e. 0 bookings).
A summary column is a good idea when you're running into performance issues with counting the # of bookings each time. In that case I recommend creating insert and delete triggers on the customer_booking table that either increment or decrement the number_of_bookings column.

Answer 2

You could do it in a single straight select like this:
select DISTINCT
b1.room_pk,
c1.no_of_bookings
from cust_bookings b1,
(select room_pk, count(1) as no_of_bookings
from cust_bookings
group by room_pk) c1
where b1.room_pk = c1.room_pk
having c1.no_of_bookings < 3
Sorry i used my own table names to test it but you should figure it out easily enough. Also, the "having" line is only there to limit the rows returned to rooms with less than 3 bookings. If you remove that line you will get everything and could use the same sql to update a column on the bookings table if you still want to go that route.

Answer 3

Consider below solutions.
A simple aggregate query to count the customers per each booking:
SELECT b.PK_room_number, Count(c.FK_customer_id)
FROM Booking b
INNER JOIN Customer_Booking c ON b.PK_room_number = c.FK_room_number
GROUP BY b.PK_room_number
HAVING Count(c.FK_customer_id) < 3; # ADD 3 ROOM MAX FILTER
And if you intend to use a new column no_of_booking, here is an update query (using aggregate subquery) to run right after inserting new value from web frontend:
UPDATE Booking b
INNER JOIN
(SELECT b.PK_room_number, Count(c.FK_customer_id) As customercount
FROM Booking b
INNER JOIN Customer_Booking c ON b.PK_room_number = c.FK_room_number
GROUP BY b.PK_room_number) As r
ON b.PK_room_number = r.PK_room_number
SET b.no_of_booking = r.customercount;

Answer 4

the following generates a list showing all of the bookings and a flag of 0 or 1 if the the room has a customer for each of the rooms. it will display some rooms multiple times if there are multiple customers.
select BOOKING.*,
case CUSTOMER_BOOKING.FK_ROOM_NUMBER is null THEN 0 ELSE 1 END AS BOOKING_FLAG
from BOOKING LEFT OUTER JOIN CUSTOMER_BOOKING
ON BOOKING.PK_room_numer = CUSTOMER_BOOKING.FK_room_number
summing and grouping we arrive at:
select BOOKING.*,
SUM(case when CUSTOMER_BOOKING.FK_ROOM_NUMBER is null THEN 0 ELSE 1 END) AS BOOKING_COUNT
from BOOKING LEFT OUTER JOIN CUSTOMER_BOOKING
ON BOOKING.PK_room_number = CUSTOMER_BOOKING.FK_room_number
GROUP BY BOOKING.PK_room_number
there are at least two other solutions I can think of off the top of my head...