将新数据添加到数据库时运行SQL Server触发器

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATETIME
);

CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    CustomerName VARCHAR(100),
    TotalOrders INT DEFAULT 0
);

CREATE TRIGGER trg_AfterInsertOrder
ON Orders
AFTER INSERT
AS
BEGIN
    UPDATE Customers
    SET TotalOrders = TotalOrders + 1
    WHERE CustomerID IN (SELECT CustomerID FROM INSERTED);
END;

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    EmployeeName VARCHAR(100)
);

CREATE TRIGGER trg_InsteadOfInsertEmployee
ON Employees
INSTEAD OF INSERT
AS
BEGIN
    IF NOT EXISTS (SELECT EmployeeID FROM Employees WHERE EmployeeID IN (SELECT EmployeeID FROM INSERTED))
    BEGIN
        INSERT INTO Employees (EmployeeID, EmployeeName)
        SELECT EmployeeID, EmployeeName FROM INSERTED;
    END
    ELSE
    BEGIN
        RAISERROR ('Employee ID already exists!', 16, 1);
    END
END;