Find DISTINCT LAST record with SQL LEFT JOIN - mysql

Question

I'm running MySQL 5.6.
I have two related tables:
CREATE TABLE Cars (
id INT NOT NULL AUTO_INCREMENT,
plate VARCHAR(16) NOT NULL,
flag TINYINT,
PRIMARY KEY(id)
)
and:
CREATE TABLE Rents (
id INT NOT NULL AUTO_INCREMENT,
out_date DATE NOT NULL,
in_date DATE,
car_id INT,
FOREIGN KEY (car_id) REFERENCES Cars(id),
PRIMARY KEY(id)
)
I can have multiple rents for each car (0 to many).
I need to select all vehicles in table Cars (with flag = 1) along with their status i.e. I need to know if each car is currently unavailable (only out_date is filled) or availabe (out_date and in_date filled) of course also vehicles without any rents are to be considered available.
The result set need to include out_date and in_date values [Update 17/07/2022].
I tought to use something like:
SELECT
*,
IF(Rents.in_date IS NOT NULL AND Rents.out_date IS NOT NULL, 1, IF(Rents.id IS NULL, 1, 0)) AS status
FROM Cars
LEFT JOIN Rents ON Cars.id = Rent.Car_id WHERE Cars.Flag = 1
but this of course will just return all the rows with positive flag match and a status evaluation (0 unavailable, 1 available):
id | plate | flag | id | out_date | in_date | car_id | status
---------------------------------------------------------------------
'1', 'FA787MX', '1', '1', '2022-07-14', '2022-07-15', '1', '1'
'1', 'FA787MX', '1', '2', '2022-07-16', NULL, '1', '0'
'3', 'AB124DF', '1', '4', '2022-07-13', '2022-07-14', '3', '1'
'4', 'CC666VC', '1', NULL, NULL, NULL, NULL, '1'
'5', 'GG435ED', '1', '5', '2022-07-16', NULL, '5', '0'
While I need to have this (edited 17/07/2022):
'1', 'FA787MX', '1', '2', '2022-07-16', NULL, '1', '0'
'3', 'AB124DF', '1', '4', '2022-07-13', '2022-07-14', '3', '1'
'4', 'CC666VC', '1', NULL, NULL, NULL, NULL, '1'
'5', 'GG435ED', '1', '5', '2022-07-16', NULL, '5', '0'
i.e. only the second row of FA787MX car should be mantained since it's the most recent out_date value (no matter if it's id is higher or lower).
For the sake of completeness: There is no guarantee that rental ids will be kept consistent with their rental history. In other words you cannot be sure that for a given car the rental where in_date = NULL is the correct one but you should compare them by out_date value.
Data sample:
INSERT INTO `Cars` (`id`, `plate`, `flag`) VALUES (1, 'FA787MX', 1);
INSERT INTO `Cars` (`id`, `plate`, `flag`) VALUES (2, 'EX431YY', 0);
INSERT INTO `Cars` (`id`, `plate`, `flag`) VALUES (3, 'AB124DF', 1);
INSERT INTO `Cars` (`id`, `plate`, `flag`) VALUES (4, 'CC666VC', 1);
INSERT INTO `Cars` (`id`, `plate`, `flag`) VALUES (5, 'GG435ED', 1);
INSERT INTO `Rents` (`id`, `out_date`, `in_date`, `car_id`) VALUES (1, '2022-07-14', '2022-07-15', 1);
INSERT INTO `Rents` (`id`, `out_date`, `in_date`, `car_id`) VALUES (2, '2022-07-16', NULL, 1);
INSERT INTO `Rents` (`id`, `out_date`, `in_date`, `car_id`) VALUES (3, '2022-07-16', NULL, 2);
INSERT INTO `Rents` (`id`, `out_date`, `in_date`, `car_id`) VALUES (4, '2022-07-13', '2022-07-14', 3);
INSERT INTO `Rents` (`id`, `out_date`, `in_date`, `car_id`) VALUES (5, '2022-07-16', NULL, 5);

Answer 1

One option is to join to find only those rentals that are still outstanding (in_date IS NULL). That will drop the old rentals having in_date not null.
Based on the updated requirements, there are a few ways to do it. One is a simple outer join to find the most recent rental per car to obtain the corresponding in_date as well...
MySQL 5.6 fiddle
SELECT Cars.*
, Rents.out_date
, Rents.in_date
, Rents.id IS NULL OR Rents.in_date IS NOT NULL AS status_final
FROM Cars
LEFT JOIN Rents
ON Cars.id = Rents.Car_id
LEFT JOIN Rents AS r2
ON Rents.out_date < r2.out_date
AND Rents.Car_id = r2.Car_id
WHERE Cars.Flag = 1
AND r2.Car_id IS NULL
ORDER BY Cars.id
;
The result:
id
plate
flag
out_date
in_date
status_final
1
FA787MX
1
2022-07-16
0
3
AB124DF
1
2022-07-13
2022-07-14
1
4
CC666VC
1
1
5
GG435ED
1
2022-07-16
0
Based on the original requirements: Try this (fiddle):
SELECT Cars.*
, Rents.in_date
, CASE WHEN in_date IS NOT NULL OR Rents.id IS NULL THEN 1 ELSE 0 END AS status_final
FROM Cars
LEFT JOIN Rents
ON Cars.id = Rents.Car_id
AND in_date IS NULL
WHERE Cars.Flag = 1
;
and if the results contain only those with in_date IS NULL, this reduces to:
SELECT Cars.*
, out_date
, Rents.in_date
, Rents.id IS NULL AS status_final
FROM Cars
LEFT JOIN Rents
ON Cars.id = Rents.Car_id
AND in_date IS NULL
WHERE Cars.Flag = 1
;
Result:
id
plate
flag
out_date
in_date
status_final
1
FA787MX
1
2022-07-16
0
3
AB124DF
1
1
4
CC666VC
1
1
5
GG435ED
1
2022-07-16
0

Answer 2

If your version of MySql is 8.0+ use ROW_NUMBER() window function to pick the latest row for each car in Rents:
SELECT c.*, r.*,
r.out_date IS NULL OR r.in_date IS NOT NULL status
FROM Cars c
LEFT JOIN (
SELECT *, ROW_NUMBER() OVER (PARTITION BY car_id ORDER BY out_date DESC) rn
FROM Rents
) r ON r.car_id = c.id AND r.rn = 1
WHERE c.flag = 1;
For previous versions use NOT EXISTS:
SELECT c.*, r.*,
r.out_date IS NULL OR r.in_date IS NOT NULL status
FROM Cars c
LEFT JOIN (
SELECT r1.*
FROM Rents r1
WHERE NOT EXISTS (
SELECT *
FROM Rents r2
WHERE r2.car_id = r1.car_id AND r2.out_date > r1.out_date
)
) r ON r.car_id = c.id
WHERE c.flag = 1;
See the demo.

Answer 3

If you imagine the result of your query as a table, you can easily write a query that would give you what you need (the subquery is just yours with the select spelled out to give a unique column name to the second id column, as it seemed useful - the only way to uniquely identify a row):
SELECT MAX(rent_id) FROM (
SELECT
Cars.id as id,
plate,
flag,
Rents.id as rent_id,
out_date,
in_date,
car_id,
IF(Rents.in_date IS NOT NULL AND Rents.out_date IS NOT NULL, 1, IF(Rents.id IS NULL, 1, 0)) AS status
FROM Cars
LEFT JOIN Rents ON Cars.id = Rents.car_id WHERE Cars.Flag = 1
) as rental_status
WHERE status = 0
GROUP BY car_id;
Which tells you which rows are interesting:
+--------------+
| MAX(rent_id) |
+--------------+
| 2 |
| 5 |
+--------------+
Now you can use a join to return the results of your initial query only for the interesting rows. To avoid having to spell out that query all over again, MySQL 8 has a way to stash the results of your core query and use it like a table:
WITH
status_data AS (
SELECT
Cars.id as id,
plate,
flag,
Rents.id as rent_id,
out_date,
in_date,
car_id,
IF(Rents.in_date IS NOT NULL AND Rents.out_date IS NOT NULL, 1, IF(Rents.id IS NULL, 1, 0)) AS status
FROM Cars
LEFT JOIN Rents ON Cars.id = Rents.car_id WHERE Cars.Flag = 1
)
SELECT * from status_data
JOIN (
SELECT MAX(rent_id) as rent_id FROM status_data
WHERE status = 0
GROUP BY car_id
) as ids using(rent_id);
Giving the result:
+---------+----+---------+------+------------+---------+--------+--------+
| rent_id | id | plate | flag | out_date | in_date | car_id | status |
+---------+----+---------+------+------------+---------+--------+--------+
| 2 | 1 | FA787MX | 1 | 2022-07-16 | NULL | 1 | 0 |
| 5 | 5 | GG435ED | 1 | 2022-07-16 | NULL | 5 | 0 |
+---------+----+---------+------+------------+---------+--------+--------+