如何创建一个新值我们的行数乘以另一个值

Question

我想在CruisePrice乘客次数之外创建一个新值TotalPrice。我有以下不起作用：

SELECT PlaceResidency, BirthDate, LastName, FirstName
    , (SELECT count (*) AS "PassengerAmount" FROM PASSENGER)
    , PassengerAmount * CruisePrise AS TotalPrice
FROM PASSENGER, PERSON, CRUISE 
WHERE BirthDate >= 18
ORDER BY PlaceResidency, BirthDate, TotalPrice;

Answer 1

您应该声明一个变量来保存乘客数量，并在需要的地方使用它以避免在子查询中重新计算。

DECLARE @PassengersCount INT = (SELECT count (*)  FROM PASSENGER);

SELECT PlaceResidency, BirthDate, LastName, FirstName, @PassengersCount AS 
PassengerAmount, @PassengersCount * CruisePrise AS TotalPrice
FROM PERSON, CRUISE
WHERE BirthDate >= 18
ORDER BY PlaceResidency, BirthDate, TotalPrice;

Answer 2

请尝试这个解决方案可能有效：

SELECT PlaceResidency, BirthDate, LastName, FirstName
    , (SELECT count (*)  FROM PASSENGER) AS PassengerAmount
    , ((SELECT count (*)  FROM PASSENGER) * CruisePrise )AS TotalPrice
FROM PASSENGER, PERSON, CRUISE 
WHERE BirthDate >= 18
ORDER BY PlaceResidency, BirthDate, TotalPrice;