在SqlServer中模拟Oracle Before触发器

1. 插入数据前判断数据是否存在

在 Oracle 中，Before Trigger 常用于在进入主逻辑前进行业务规则校验。以下 SQL Server 脚本通过 Instead of insert 触发器实现了类似的功能：当插入操作发生时，若指定分类名称已存在，则阻止插入并输出提示；否则执行实际插入操作。

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:     <Author,,Name>
-- Create date: <Create Date,,>
-- Description: --<Description,,>
-- =============================================
ALTER TRIGGER CategoryExistTrigger
ON ProductCategory
INSTEAD OF INSERT
AS
BEGIN
  DECLARE @CategoryName VARCHAR(50);

  SET NOCOUNT ON;

  -- 提取被插入的记录
  SELECT @CategoryName = CategoryName FROM inserted;

  -- 如果记录已存在则中断并打印信息
  IF EXISTS (SELECT * FROM ProductCategory WHERE CategoryName = @CategoryName)
  BEGIN
    PRINT 'Category exists..';
  END
  ELSE
  BEGIN
    -- 数据不存在时执行插入
    INSERT INTO ProductCategory SELECT * FROM inserted;
  END
END;

2. 删除表中数据时需要先删除外键表的数据

当父表中有子表引用其主键时，直接删除会导致外键约束报错。在 Sql Server 中，可通过 Instead of delete 触发器捕获删除请求，并在实际删除前按规则清理关联数据。

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:     <Author,,Name>
-- Create date: <Create Date,,>
-- Description: --<Description,,>
-- =============================================
ALTER TRIGGER DeleteOrderTrigger
ON OrderHeader
INSTEAD OF DELETE
AS
BEGIN
  SET NOCOUNT ON;

  DECLARE @OrderId VARCHAR(50);
  SELECT @OrderId = OrderId FROM deleted;

  -- 先删除该订单关联的行项记录
  DELETE FROM OrderLine WHERE OrderId = @OrderId;

END
GO;