触发器如何判断是插入，是删除，还是更新

create trigger trig_action on tb_teacher


for insert , update , delete  -- 带有 FOR 关键字的 DML 触发器也归类为 AFTER 触发器
AS
BEGIN
SET NOCOUNT ON;
if not exists(select 1 from deleted)
   insert into Action_Log(TableName,ActionName,ActionTime) values('tb_teacher','插入操作',getdate());
else if
   not exists (select 1 from inserted)
   insert into Action_Log(TableName,ActionName,ActionTime) values('tb_teacher','删除操作',getdate());
else 
   insert into Action_Log(TableName,ActionName,ActionTime) values('tb_teacher','更新操作',getdate());
END
GO

-- update 操作其实就是 delete + insert 所以在 exists(select 1 from deleted)并不是说就是删除操作