code: private Object executeConstant(SQL sql, Query qr, ZConstant constant, int row) throws PageException {
  	switch (constant.getType()) {
  	case ZConstant.COLUMNNAME: {
  	    if (constant.getValue().equals(SQLPrettyfier.PLACEHOLDER_QUESTION)) {
  		int pos = sql.getPosition();
  		sql.setPosition(pos + 1);
  		if (sql.getItems().length <= pos) throw new DatabaseException("invalid syntax for SQL Statement", null, sql, null);
  		return sql.getItems()[pos].getValueForCF();
  	    }
  	    return qr.getAt(ListUtil.last(constant.getValue(), ".", true), row);
  	}
  	case ZConstant.NULL:
  	    return null;
  	case ZConstant.NUMBER:
  	    return Caster.toDouble(constant.getValue());
  	case ZConstant.STRING:
  	    return constant.getValue();
  	case ZConstant.UNKNOWN:
  	default:
  	    throw new DatabaseException("invalid constant value", null, sql, null);
  	}
      }
docs: Executes a constant value
  
  @param sql
  @param qr
  @param constant
  @param row
  @return result
  @throws PageException