virtual datapoint - how to preform expression valuation if statement

What you need is a nested if statement.

if( un(DatapointX), 0, if( gt(DatapointX,0),1, -1))

This means:

if(
  un(DatapointX),   <-- this is your condition (conditionA)
  0,                         <-- return this if conditionA is true
  if(                          <-- do another if statement if conditionA is false
    gt(DatapointX,0),  <--We know conditionA is false, let's do another condition (conditionB) , in this case if DatapointX is greater than 0.
    1,                          <--return this if conditionB is true
    2                         <-- return this if conditionB is false
    )
  )