$P{theParameter} == "value_1" ?
     (" AND CONDITION_1 IN ('A', 'B', 'C') AND CONDITION_2 = 'Yes' "
     ) : " AND CONDITION_3 = 'Other' AND CONDITION_4 = 'No' "

WHERE
      .... = .....
      AND .... = ....
      AND .... = ....
      $P!{theParameter}

WHERE
      .... = .....
      AND .... = ....
      AND .... = ....
      AND thisValue = $P!{theParameter}