code: protected int addFieldExpr(final String _expr)
      {
          int ret = -1;
          if (getAllFieldExpr().containsKey(_expr)) {
              ret = getFieldExprIndex(_expr);
          } else {
              getAllFieldExpr().put(_expr, Integer.valueOf(getSelIndexLen()));
              if (getSelect() == null) {
                  setSelect(_expr);
              } else {
                  setSelect(getSelect() + "," + _expr);
              }
              ret = getSelIndexLen();
              this.selIndexLen++;
          }
          return ret;
      }
docs: Add a field expression to the select statement and the hash table of all
  field expressions. The method returns the index of the field expression.
  If the field expression is already added, the old index is returned, so a
  expression is only added once.
  
  @param _expr field expression to add
  @return index of the field expression
  @see #getFieldExprIndex
  @see #getAllFieldExpr
  @see #allFieldExpr