不明白触发器是如何工作的

Question

触发器应该对从 table 供应商中删除行做出反应，如果 table 供应商的供应中有一些供应，触发器应该取消删除。此代码允许同时删除有供应的供应商和没有供应的供应商：

CREATE TRIGGER SuppliersDeleteCondition
ON Suppliers
FOR DELETE
AS
IF EXISTS (
           SELECT 1
             FROM Supplies Ses
             JOIN DELETED D
               ON D.SupplierID=Ses.SupplierID
          )
BEGIN
  RAISERROR ('This supplier has some supplies', 16, 1)
  ROLLBACK TRANSACTION
END;

大约 table 的观点是：

供应商（供应商 ID、名称、地址、银行详细信息）、供应（供应商 ID、供应商 ID、产品 ID、实施期、重量、价格）

这里是创建 TABLE:

CREATE TABLE Suppliers
        (
         SupplierID INT IDENTITY,
         Name VARCHAR(150) NOT NULL UNIQUE,
         Address VARCHAR(900) NOT NULL,
         BankDetails VARCHAR(9) NOT NULL UNIQUE,

         CONSTRAINT pk_SupplierID PRIMARY KEY (SupplierID),
         CONSTRAINT chk_Name_Suppliers CHECK (NOT Name LIKE '%[^a-z ]%' AND NOT Name LIKE '[ ]%' AND NOT Name LIKE '%[ ]' AND NOT Name LIKE '%[ ][ ]%'),
         CONSTRAINT chk_Address_Suppliers CHECK (NOT Address LIKE '%[^a-zA-z0-9,./ ]%' AND NOT Address LIKE '[ ]%'),
         CONSTRAINT chk_BankDetails_Suppliers CHECK (BankDetails LIKE '[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]')
        );
CREATE TABLE Supplies
        (
         SupplyID INT IDENTITY,
         SupplierID INT,
         ProductID INT,
         ImplementationPeriod DATE NOT NULL,
         Weight REAL NOT NULL,
         Price MONEY NOT NULL,

         CONSTRAINT pk_SupplyID PRIMARY KEY (SupplyID),
         CONSTRAINT fk_SupplierID FOREIGN KEY (SupplierID) REFERENCES Suppliers(SupplierID) ON DELETE CASCADE,
         CONSTRAINT fk_ProductID FOREIGN KEY (ProductID) REFERENCES Products(ProductID) ON DELETE CASCADE,
         CONSTRAINT chk_Weight_Supplies CHECK (Weight > 0)
        );

当我这样删除行时：

DELETE FROM Suppliers WHERE SupplierID=18

此行已删除，我没有收到任何错误

Answer 1

您有一个带有 DELETE CASCADE 选项的外键，因此删除触发器是多余的。 AFTER DELETE 触发器将在供应商和用品的行被删除后触发，因此没有行符合 EXISTS 谓词的条件。除非您可以删除外键，否则这里不需要触发器，我不推荐这样做。让 SQL 服务器为您完成工作。

不明白触发器是如何工作的

don`t understand how trigger works

sql

sql-server

ssms