Oracle-Making Optional Constraint in Select : How to make a constraint optional?
That is, if a codition of the consraint is FALSE, how to ignore the constraint in a simplest way?
The expected result of my code is,

    RR
    --
    100
    200

My Code is,

    WITH DATASET AS ( 
      SELECT 1 A, 10 B, 100 RR FROM DUAL
      UNION
      SELECT NULL A, 10 B, 200 RR FROM DUAL
    )
    SELECT RR
    FROM DATASET
    WHERE 
    A=2  -- How to make this constraint optional(that is, if this is FALSE,ignoring this constraint?)
    AND 
    B=10 
    ;