当满足特定值时在 SQL 中指定值的限制

Question

在 SQL 中，如何在满足另一个条件时为属性值设置限制？ 例如，一个学生最多可以借 3 个资源，教职员工最多可以借 6 个

Create Table USER_T
(   UserID int PRIMARY KEY NOT NULL,
    UserType varchar (25) NOT NULL,
    No_of_Resources_Borrowed int check (No_of_Resources_Borrowed=<6)
    CONSTRAINT con_user_typ check (UserType IN ('Student','Staff'))
);

Answer 1

你想要一个 CONSTRAINT 子句

Create Table USER_T
(   UserID int PRIMARY KEY NOT NULL,
    UserType varchar (25) NOT NULL,
    No_of_Resources_Borrowed int NOT NULL,
    CONSTRAINT Resources_by_type CHECK (
      No_of_Resources_Borrowed 
      <= case UserType when 'Student' then 3 when 'Staff' then 6 else 2147483647 end ),
    CONSTRAINT con_user_typ CHECK (UserType IN ('Student','Staff'))
    );