我们有一个用户表，每个用户都有一个唯一的电子邮件和用户名。我们尝试在我们的代码中执行此操作，但是我们希望确保用户不会以相同的电子邮件用户名插入（或更新）到数据库中。我添加了一个BEFORE INSERT触发器，以防止插入重复的用户。

CREATE TRIGGER [dbo].[BeforeUpdateUser]
   ON  [dbo].[Users]
   INSTEAD OF INSERT
AS 
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    DECLARE @Email nvarchar(MAX)
    DECLARE @UserName nvarchar(MAX)
    DECLARE @UserId int
    DECLARE @DoInsert bit

    SET @DoInsert = 1

    SELECT @Email = Email, @UserName = UserName FROM INSERTED

    SELECT @UserId = UserId FROM Users WHERE Email = @Email

    IF (@UserId IS NOT NULL)
        BEGIN
            SET @DoInsert = 0
        END

    SELECT @UserId = UserId FROM Users WHERE UserName = @UserName

    IF (@UserId IS NOT NULL)
        BEGIN
            SET @DoInsert = 0
        END

    IF (@DoInsert = 1)
        BEGIN
            INSERT INTO Users
            SELECT 
                           FirstName, 
                           LastName, 
                           Email, 
                           Password, 
                           UserName, 
                           LanguageId,
                           Data, 
                           IsDeleted 
                       FROM INSERTED
        END
    ELSE
        BEGIN
            DECLARE @ErrorMessage nvarchar(MAX)
            SET @ErrorMessage = 
                         'The username and emailadress of a user must be unique!'
            RAISERROR 50001 @ErrorMessage
        END 
END

但是对于更新触发器，我不知道如何执行此操作。我在Google上找到了以下示例：http : //www.devarticles.com/c/a/SQL-
Server/Using-Triggers-In-MS-SQL-Server/2/ 但我不知道它是否适用一次更新多个列时。

编辑：

我试图在这些列上添加唯一约束，但是它不起作用：

Msg 1919, Level 16, State 1, Line 1
Column 'Email' in table 'Users' is of a type 
that is invalid for use as a key column in an index.