SQL-如何使用联接在同一表上执行交集（带有子查询）？

Question

I have two tables. Car table, and rental table.

Car:

id brand
--------
0  honda
1  yota

Rental:

id id_car start end
--------
1  1      1     2
2  1      2     NULL

I need to execute two queries. One that shows all cars that are rented. The other that shows all cars that are NOT rented.

The first query was easy for me.

SELECT car.id, car.brand, reg.start, reg.end
FROM car
LEFT JOIN car_registration AS reg ON car.id = reg.id_car
WHERE reg.end IS NULL

The reverse query (for determining rented cars) would involve something along the lines of the opposite. Find a list of rentals, where reg.id_car = car.idm where reg.end IS NOT NULL. However, if an entry exists in the same table where reg.end IS NULL, then the car is actually rented. I need to intersect a list of car rentals where reg.end IS NULL and renteals where reg.end IS NOT NULL.

Pseudo-sql would be something like this.

Select DISTINCT car.id (so that duplicate historical rentals are not returned), join with car rentals table, where reg.end is NOT NULL. This would give me all cars that are not rented (expired rentals). However, I need to intersect this with a list of car rentals where reg.end IS null, in order to determine that the car is in-fact not rented.

I know SQL doesn't have an intersect statement. I get how joins work in principle, but this is confusing the hell out of me. Any ideas?

Answer 1

You can get both rented and unrented cars in a single query by simply left joining the car table against a subselect where you determine cars that are currently rented out.

SELECT
    c.id AS id,
    c.brand AS brand,
    r.rented AS rented
FROM
    (
        SELECT
            id,
            brand
        FROM car
    ) AS c
LEFT JOIN
    (
        SELECT DISTINCT
            id_car,
            1 AS rented
        FROM car_registration
        WHERE `end` IS NULL
    ) AS r
ON c.id = r.id_car

All rows where you have 1 for rented would signify cars that are currently being rented out.