如何在查询中的查询中使用 ISNULL？

Question

下面的 SQL 查询现在可以正确返回 RoomsAvailable 字段，除非有 0 个房间被占用，其中 SQL 命令用 NULL 减去 number_of_rooms 并将 NULL 输出到列。我尝试了许多 ISNULL 变体，发现它不起作用；有人知道我应该怎么做吗？

SQL：

SELECT
    Hotel_2.hotel_code, 
    Hotel_2.hotel_country, 
    Room_type_rates_2.room_type_code, 
    Room_type_rates_2.number_of_rooms, 
    Types_2.room_type, 
    Room_type_rates_2.rates, 
            Room_type_rates_2.number_of_rooms -
            (SELECT 
                DISTINCT (SELECT
                             COUNT(dbo.Hotel.hotel_code) AS RoomsTake
                          FROM
                              dbo.Hotel 
                          INNER JOIN dbo.Hotel_Reservation 
                              ON dbo.Hotel.hotel_code = dbo.Hotel_Reservation.hotel_code 
                          INNER JOIN dbo.Room_type_rates 
                              ON dbo.Hotel.hotel_code = dbo.Room_type_rates.hotel_code 
                          INNER JOIN dbo.Types 
                              ON dbo.Hotel_Reservation.room_type_code = dbo.Types.room_type_code 
                              AND dbo.Room_type_rates.room_type_code = dbo.Types.room_type_code
                          WHERE
                              (dbo.Room_type_rates.room_type_code = Room_type_rates_1.room_type_code) 
                              AND (dbo.Hotel.hotel_code = Hotel_1.hotel_code)
                              AND (dbo.Hotel_Reservation.checkin_date >= Hotel_Reservation_1.checkin_date)
                              AND (dbo.Hotel_Reservation.checkout_date <= Hotel_Reservation_1.checkout_date)
                          ) AS RoomsTaken
               FROM
                   dbo.Hotel AS Hotel_1 
               INNER JOIN dbo.Hotel_Reservation AS Hotel_Reservation_1
                   ON Hotel_1.hotel_code = Hotel_Reservation_1.hotel_code 
               INNER JOIN dbo.Room_type_rates AS Room_type_rates_1 
                   ON Hotel_1.hotel_code = Room_type_rates_1.hotel_code 
               INNER JOIN dbo.Types AS Types_1 
                   ON Hotel_Reservation_1.room_type_code = Types_1.room_type_code 
                   AND Room_type_rates_1.room_type_code = Types_1.room_type_code
               WHERE
                   (Hotel_Reservation_1.checkin_date >= '11/19/2011') 
                   AND (Hotel_Reservation_1.checkout_date <= '12/01/2011') 
                   AND (Hotel_1.hotel_country = 'Adelaide') 
                   AND (Types_1.room_type_code = Types_2.room_type_code)
               ) AS RoomsAvailable 
FROM 
    dbo.Hotel AS Hotel_2 
INNER JOIN dbo.Room_type_rates AS Room_type_rates_2 
    ON Hotel_2.hotel_code = Room_type_rates_2.hotel_code 
INNER JOIN dbo.Types AS Types_2 
    ON Room_type_rates_2.room_type_code = Types_2.room_type_code

当前输出：

<代码> ADL20 阿德莱德 CPL 6 情侣套房 514.0000 3

ADL20   Adelaide    FYU         3   Family Suite    533.0000    2

ADL20   Adelaide    KNG         2   King's Bedroom  556.0000    NULL

Answer 1

你的意思是这个吗？：

...ISNULL(COUNT(dbo.Hotel.*),0)...

或者你根本就得到了NULL？在这种情况下，我怀疑您的 JOIN 可能有问题。

您可以将 ISNULL 包裹在任何嵌套子查询或值周围。