What does this ?? notation mean here

Answer 1

It means "if id or customerID is null, pretend it's "ALFKI" instead.

Answer 2

It's the null coalescing operator: http://msdn.microsoft.com/en-us/library/ms173224(VS.80).aspx

It provides a value (right side) when the first value (left side) is null.

Answer 3

That's the null-coalescing operator.

var x = y ?? z;

// is equivalent to:
var x = (y == null) ? z : y;

// also equivalent to:
if (y == null) 
{
    x = z;
}
else
{
    x = y;
}

ie: x will be assigned z if y is null, otherwise it will be assigned y.
So in your example, customerID will be set to "ALFKI" if it was originally null.