Disallow a duplicate entry across two columns in a SQL CE database

Hi,

I have 4 columns in a table (rates) of my db  viz. SrKey (int), ItemID (int) {Related with another table}, DateValidFrom (datetime), Rate(double)

I want to disallow the user from adding a new rate for a particular item on the same date.

for eg. if there is a rate say $20 for ItemID 12, valid from 09/26/2012, I want to disallow a user from entering another rate valid from 09/26/2012 for the ItemID 12. but he can enter a new rate for another item (provided he hasn't entered a rate for that item on the same date)

I am using SQL CE 4. VB 2010 express

Please help

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