>Hi Sergey, I have it sussed now, this is what I have
>
>

>@MyFlag = 'Y'
>
>update mytable
>set field1 = null
>where @MyFlag = 'Y' And MasterCustomerRef = @MasterCustomerRef
>Or @MyFlag = 'N' And CustomerRef = @CustomerRef
>
>

>
>maybe not pretty but it works
>

As I said, I'm not sure why it ahs to be one update. Also, when you combine OR and AND it's better to use parenthesis

where (@MyFlag = 'Y' And MasterCustomerRef = @MasterCustomerRef)
Or (@MyFlag <> 'Y' And CustomerRef = @CustomerRef)