Mysql search query - php

Question

I am developing a car rental site. I have two tables test_tbl_cars and test_reservations.
I am using the search query (cribbed from Jon Kloske in "How do I approach this PHP/MYSQL query?"):
$sql = mysql_query("SELECT
test_tbl_cars.*,
SUM(rental_start_date <= '$ddate' AND rental_end_date >= '$adate') AS ExistingReservations
FROM test_tbl_cars
LEFT JOIN test_reservations USING (car_id)
GROUP BY car_id
HAVING ExistingReservations = 0");
This gives me excellent search results but the test_tbl_cars table contains many cars which in any given search returns several of the same car model as being available.
How can I filter the query return such that I get one of each model available?

Answer 1

Use Distict clause
$sql = mysql_query("SELECT
DISTINCT test_tbl_cars.model, test_tbl_cars.*,
SUM(rental_start_date <= '$ddate' AND rental_end_date >= '$adate') AS ExistingReservations
FROM test_tbl_cars
LEFT JOIN test_reservations USING (car_id)
GROUP BY car_id
HAVING ExistingReservations = 0");

Answer 2

Awww, should have tagged me, I only saw this now over a year later! You've probably already figured out how to work this by now, but I'll take a crack at it anyway for completeness sake and because most of the answers here I don't think are doing what you want.
So the problem you are having is that in the other question each room had a unique ID and it was unique rooms people were interested in booking. Here, you're extending the concept of a bookable item to a pool of items of a particular class (in this case, model of car).
There's may be a way to do this without subqueries but by far the easiest way to do it is to simply take the original idea from my other answer and extend it by wrapping it up in another query that does the grouping into models (and as you'll see shortly, we get a bunch of other useful stuff for free out of doing this).
So, firstly lets start by getting the list of cars with counts of conflicting reservations (as per the update to my other answer):
(I'll use your query for these examples as a starting point, but note you really should use prepared statements or at the very least escaping functions supplied by your DB driver for the two parameters you're passing)
SELECT car_id, model_id, SUM(IF(rental_id IS NULL, 0, rental_start_date <= '$ddate' AND rental_end_date >= '$adate')) AS ConflictingReservations
FROM test_tbl_cars
LEFT JOIN test_reservations USING (car_id)
GROUP BY car_id
This will return one row per car_id giving you the model number, and the number of reservations that conflict with the date range you've specified (0 or more).
Now at this stage if we were asking about individual cars (rather than just models of cars available) we could restrict and order the results with "HAVING ConflictingReservations = 0 ORDER BY model_id" or something.
But, if we want to get a list of the availability of ~models~, we need to perform a further grouping of these results to get the final answer:
SELECT model_id, COUNT(*) AS TotalCars, SUM(ConflictingReservations = 0) AS FreeCars, CAST(IFNULL(GROUP_CONCAT(IF(ConflictingReservations = 0, car_id, NULL) ORDER BY car_id ASC), '') AS CHAR) AS FreeCarsList
FROM (
SELECT car_id, model_id, SUM(IF(rental_id IS NULL, 0, rental_start_date <= '$ddate' AND rental_end_date >= '$adate')) AS ConflictingReservations
FROM test_tbl_cars
LEFT JOIN test_reservations USING (car_id)
GROUP BY car_id
) AS CarReservations
GROUP BY model_id
You'll notice all we're doing is grouping the original query by model_id, and then using aggregate functions to get us the model_id, a count of total cars we have of this model, a count of free cars of this model we have which we achieve by counting all the times a car has zero ConflictingReservations, and finally a cute little bit of SQL that returns a comma separated list of the car_ids of the free cars (in case that was also needed!)
A quick word on performance: all the left joins, group bys, and subqueries could make this query very slow indeed. The good news is the outer group by should only have to process as many rows as you have cars for, so it shouldn't be slow until you end up with a very large number of cars. The inner query however joins two tables (which can be done quite quickly with indexes) and then groups by the entire set, performing functions on each row. This could get quite slow, particularly as the number of reservations and cars increases. To alleviate this you could use where clauses on the inner query and combine that with appropriate indexes to reduce the number of items you are inspecting. There's also other tricks you can use to move the comparison of the start and end dates into the join condition, but that's a topic for another day :)
And finally, as always, if there's incorrect edge cases, mistakes, wrong syntax, whatever - let me know and I'll edit to correct!