SQL Query - uniqueness in combination

Question

Hi all,

 

I have a table as -

ID	Code	Value
1	101	abc
2	101	def
3	201	abc
4	101	def

 

Now,

 

In this table, I need to check the uniqueness in combination -

101 - abc  ---> allowed.

101 - def ---> allowed.

101 - def ->> NOT ALLOWED.... Combination "101 - def" exists.

Similarly,

Combination "101 - abc" must not be allowed next time.

 

How to check this combination uniqueness in SQL Server ??

 

Please help !!