INSERT INTO Cities VALUES(1,1)
INSERT INTO Countries (CountryId) VALUES (2)
INSERT INTO Cities VALUES(2,1)

当你更新或删除主键表时，那么外键表也会跟随一起更新或删除,需要在建表时设置级联属性

CREATE TABLE Cities( CityId INT PRIMARY KEY ,CountryId INT REFERENCES Countries ON DELETE CASCADE);
INSERT INTO Countries (CountryId) VALUES (3)

CREATE TABLE Buyers(CustomerId INT PRIMARY KEY ,CityId INT REFERENCES Cities ON DELETE CASCADE);

此外还有级联更新
INSERT INTO Countries (CountryId) VALUES (1)
on update cascade
INSERT INTO Buyers VALUES(1,1),

INSERT INTO Cities VALUES(3,2)

CREATE TABLE Countries(CountryId INT PRIMARY KEY)

复制代码 代码如下: