mysql - group results by id and print - php

Question

I have "reservation" table (mySql) that contain number of columns: res_id, hotel_id, hotel_name, from_date, to_date.
I would like to select and print html table for each hotel (i'm using PHP). the result should be a title - the name of the hotel, and bellow it a list of reservation for the specific hotel.
I can do GROUP BY:
Select * FROM reservation GROUP BY hotel_id
I'm not sure if it's the right way to do it, and how do i print the results without checking all the time if the hotel_id was changed?
Thank you in advanced

Answer 1

GROUP BY is definitely NOT the right way to approach this. One method would be:
SELECT *
FROM reservation
ORDER BY hotel_id;
You would then loop through the result sets. When the hotel name changes, you would put in the title of the hotel.
Note: This is a poor data model if it has both the hotel id and name in reservation. This would normally be in hotel and you would connect the tables using JOIN:
SELECT h.hotel_name, r.*
FROM hotels h JOIN
reservation r
ON r.hotel_id = h.hotel_id
ORDER BY hotel_id;
Using a LEFT JOIN, you can even get hotels with no reservations.

Answer 2

How is it that the hotel_id would change? As per your question it seems that hotel_id is a column made for join with a "hotels" table, isn't it?
Regarding the "group by", why would you group by hotel? This would make you loose reservations data, unless you were using some sort of group_concat.
If you want to get the reservations from a specific hotel you could loop through your hotels table and inside your loop you can do:
SELECT * FROM reservations WHERE hotel_id='QUERIED_HOTEL_ID'
Then show the results.
Or you could simply
SELECT * FROM reservations
And when you get the fetched results you can make a multidimensional php array with 'hotel_id' as top level key and 'res_id' as secondary, like this:
$reservations_by_hotel = [];
do {
$resId = $row['res_id'];
$hotelId = $row['hotel_id'];
$reservations_by_hotel[$hotelId][$resId] = $row;
} while ($row = $result->fetch_assoc());