Question

触发器应该对从表中删除行做出反应如果表中有一些耗材，则来自该供应商的耗材，触发器应取消删除。此代码允许删除供应商和供应商的供应商：

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;

大约表格视图是：

供应商（供应商ID，名称，地址，银行详细信息），供应（SupplyID，SupplierID，ProductID，ImplementationPeriod，重量，价格）

这是CREATE 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)
        );

当我删除这样的行时：

DELETE FROM Suppliers WHERE SupplierID=18

此行已删除，我没有错误

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。在供应商和耗材的行被删除后，AFTER DELETE触发器将触发，因此任何行都不会符合EXISTS谓词的条件。除非您可以删除外键，否则不需要触发器，我不建议这样做。让SQL Server为您完成工作。

不明白触发器是如何工作的

1 个答案: