在創建外鍵約束時，可以使用 ON DELETE 子句來指定當主鍵表中的記錄被刪除時外鍵表中的相應記錄應該如何處理。ON DELETE 子句可以有以下幾種選項：
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
);
