What is the best-practice for nesting PreparedStatements?

if (x !=null)
{
  SQL = "SELECT z WHERE x > y";
}
else
{
  SQL = "SELECT z WHERE x <= y";
}

SQL2 = SQL + " JOIN a ON b";

final class DatabaseQueries {
    public final static String SOME_SCENARIO       = "SELECT z WHERE x > y JOIN A, B ";
    public final static String SOME_OTHER_SCENARIO = "SELECT z WHERE x <= y JOIN A, B";
 }

PreparedStatement pstmt = getCon().prepareStatement( getQuery() );


 private String getQuery() {
     if( x != null ) {
          return DatabaseQueries.SOME_SCENARIO;
     } else {
           return DatabaseQueries.SOME_OTHER_SCENARIO;
     }
 }

final class DataBaseQueries {
    // this one is private
    private final static String JOIN_A_B = " join A, B ";
    public final static String SOME_SCENARIO       = "SELECT z WHERE x > y " + JOIN_A_B ;
    public final static String SOME_OTHER_SCENARIO = "SELECT z WHERE x <= y " + JOIN_A_B ;

}

<select id="queryName" parameterClass="com.blah.X"><!<[CDATA[
  SELECT z
  FROM a
  JOIN b ON a.id = b.foreign_key
  WHERE

  <isNotNull property="value">
    x > y
  </isNotNull>

  <isNull property="value">
    x <= y
  </isNull>

]]></select>

String selectQuery =
  (new SelectQuery())
  .addColumns(t1Col1, t1Col2, t2Col1)
  .addJoin(SelectQuery.JoinType.INNER_JOIN, joinOfT1AndT2)
  .addOrderings(t1Col1)
  .validate().toString();

String sql = "yadda yadda yadda ? yadda yadda WHERE ";
if (mystery condition 1){
   sql = sql + " page=?"
}
else if (mystery condition 2)
{
 sql = sql + " ORDER BY ? "
}

private static final String FIND_PAGE_QUERY = "...."
private static final String ORDER_BY_QUERY =" ..."

if (mystery condition 1){
   return process(FIND_PAGE_QUERY, parameters);
}
else if (mystery condition 2)
{
  return process(ORDER_BY_QUERY, parameters);
}

SELECT z WHERE (? IS NOT NULL AND x > y) OR (? IS NULL AND x <= y)