如何在SQL中的不同表中减去两列

Question

I want to enter in a query the subtract between two columns in different tables it keeps saying error ...

SELECT FlightDate,
       Plane, 
       Destination,
       Capacity
FROM  
       FlightSchedule, 
       Routes, 
       Aircrafts
WHERE 
       FlightSchedule.RID=Routes.RouteID 
       AND FlightSchedule.Plane=Aircrafts.AcID 
       AND (SELECT SUM(Capacity) 
            FROM Aircrafts) 
            - 
           (SELECT Class , count(*) 
           FROM Tickets);

Answer 1

The query is wrong on multiple levels.

First of all, you cannot put the details of SELECT capacity as a sub query under WHERE. The WHERE clause is for conditional parameters.

Try this:

WITH VACANCY as (SELECT SUM(Capacity) FROM Aircrafts) - (SELECT Count(*) FROM Tickets) 
SELECT FlightDate,Plane, Destination, Vacancy
FROM FlightSchedule,Routes,Aircrafts
WHERE  FlightSchedule.RID=Routes.RouteID AND FlightSchedule.Plane=Aircrafts.AcID;

You also need to a condition for the select count for tickets but I don't know the schema of tickets so...

Answer 2

I would split this up into 2 parts. First get the data you need, then perform the math. You are also using old SQL syntax. You should re-format using the new JOIN syntax. It's also easier to read.

1st declare a table to hold you flight schedule and aircraft info.

Declare @AIRCRAFTCAPACITY Table
(
    [FlightDate] [datetime] NOT NULL,
    [Plane] [varchar](50) NOT NULL,
    [Destination] [varchar](50) NOT NULL,
    [Capacity] [INT] NOT NULL,
    [NoOfSeatsRemaining] [INT] NULL
);

Then insert the data you need.

INSERT @AIRCRAFTCAPACITY
(
    [FlightDate],
    [Plane],
    [Destination],
    [Capacity]
)
SELECT  FS.FlightDate,
        A.Plane, 
        FS.Destination,
        A.Capacity
FROM 
        FlightSchedule FS
INNER
JOIN    Routes R
ON
        FS.RID = R.RouteID
INNER
JOIN    Aircrafts A
ON
        FS.Plane = A.AcID 

Now perform the math to calculate the remaining capacity. I've made an assumption that you are doing this for a particular route. But I'm sure you can adjust your SQL accordingly.

UPDATE  @AIRCRAFTCAPACITY
SET
        [NoOfSeatsRemaining] = [Capacity] - T.TICKETS_SOLD
FROM
        @AIRCRAFTCAPACITY A
INNER
JOIN    
        (
            SELECT ROUTEID, COUNT(ROUTEID) AS TICKETS_SOLD
            FROM
                   Tickets T1
            WHERE
                   T1.ROUTEID = A.ROUTEID
            GROUP
            BY     ROUTEID
        ) T
ON
        A.ROUTEID = T.ROUTEID

Apolgies if the syntax is a little off as it's hard to construct SQL when you dont have the underlying tables.

But hopefully it will help.