如何解决“最大允许密钥长度为900字节”。

我有以下表结构：如何解决“最大允许密钥长度为900字节”。

CREATE TABLE [dbo].[Formats] 
    (
     [caption_id] [nvarchar](512) NOT NULL, 
     Formats_exist BIT NOT NULL 
    ) 
Result-: 1 row created 

ALTER TABLE [dbo].[Formats] 
    ADD CONSTRAINT [pk__formats] 
    PRIMARY KEY CLUSTERED ([caption_id] ASC); 

Result-: 1 row created 


    CREATE TABLE [dbo].[Signature] 
    (
     [caption_id] [nvarchar](512) NOT NULL, 
     [signature_id] [int] NOT NULL 
    ) 

Result-: 1 row created 


    ALTER TABLE [dbo].[Signature] 
    ADD CONSTRAINT [pk__signatures] 
    PRIMARY KEY CLUSTERED ([caption_id] ASC,[signature_id] ASC);

Result-：

Msg 1944, Level 16, State 1, Line 1 
Index 'pk__signatures' was not created. This index has a key length of at least 1028 bytes. The maximum permissible key length is 900 bytes. 
Msg 1750, Level 16, State 0, Line 1 
Could not create constraint. See previous errors.

为什么我收到错误上面的主键，而我不是第一个收到错误虽然是nvarchar(512)和它是超过900字节。

任何人都可以帮助我解决这个问题吗？

来源

2014-02-25 Axs