在创建外键约束时,可以使用 ON DELETE 子句来指定当主键表中的记录被删除时外键表中的相应记录应该如何处理。ON DELETE 子句可以有以下几种选项:
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE CASCADE
);
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE SET NULL
);
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE RESTRICT
);
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE NO ACTION
);
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