级联删除：
on delete cascade
说明：当更新或删除主键表时，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 Countries(CountryId INT PRIMARY KEY)
INSERT INTO Countries (CountryId) VALUES (1)
INSERT INTO Countries (CountryId) VALUES (2)
INSERT INTO Countries (CountryId) VALUES (3)
CREATE TABLE Cities( CityId INT PRIMARY KEY ,CountryId INT REFERENCES Countries ON DELETE CASCADE);
INSERT INTO Cities VALUES(1,1)
INSERT INTO Cities VALUES(2,1)
INSERT INTO Cities VALUES(3,2)
CREATE TABLE Buyers(CustomerId INT PRIMARY KEY ,CityId INT REFERENCES Cities ON DELETE CASCADE);
INSERT INTO Buyers VALUES(1,1),
INSERT INTO Buyers VALUES(2,1)
INSERT INTO Buyers VALUES(3,2)
级联更新：
on update cascade