I guess you need to think the other way around. Add a column to your
select statement which using the case clause will be filled with STREET
or CITY and order on that column.

select case when porder = 1 then street else city end, .....
order by 1

If neither column is indexed anyway then you could do:
ORDER BY CASE WHEN pOrder = 1 THEN address ELSE city END

Cheers
Serge

You missed the point thaty CASE is an expression and NOT a procedural
control statement.

Standard SQL-92 does not allow you to use a function or expression in
an ORDER BY clause.  The ORDER BY clause is part of a cursor and it can
only see the column names that appear in the SELECT clause list that
was used to build the result set.  Someone will now chime in that
SQL-99 (officially called "a standard in progress" and not recognized
by the U.S. Government for actual use) does allow this.

But aside from this, there is the good programming practice of showing
the fields that are used for the sort to the user, usually on the left
side of each line since we read left to right.

The standard trick for picking a sorting order at run time is to use a
flag in CASE expression.  If you want to sort on more than one column
and allow all possible combinations of sorting use one CASE per column:

SELECT
CASE @flag_1
      WHEN 'a' THEN CAST (a AS CHAR(n))
      WHEN 'b' THEN CAST (b AS CHAR(n))
      WHEN 'c' THEN CAST (c AS CHAR(n))
      ELSE NULL END AS sort_1,
CASE @flag_2
      WHEN 'x' THEN CAST (x AS CHAR(n))
      WHEN 'y' THEN CAST (y AS CHAR(n))
      WHEN 'z' THEN CAST (z AS CHAR(n))
      ELSE NULL END AS sort_2,
       ...
CASE @flag_n
      WHEN 'n1' THEN CAST (n1 AS CHAR(n))
      WHEN 'n2' THEN CAST (n2 AS CHAR(n))
      WHEN 'n3' THEN CAST (n3 AS CHAR(n))
      ELSE NULL END AS sort_2,

 FROM Foobar
WHERE ...
ORDER BY sort_1, sort_2, ...

More than one sort column and only a limited set of combinations then
use concatenation.

CASE @flag_1
      WHEN 'ab'
      THEN CAST(a AS CHAR(n)) ||' ' || CAST(b AS CHAR(n))
      WHEN 'ba'
      THEN CAST(b AS CHAR(n)) ||' ' || CAST(a AS CHAR(n))
      ELSE NULL END AS sort_1,

If you need ASC and DESC options, then use a combination of CASE and
ORDER BY

CASE @flag_1
      WHEN @flag_1 = 'a' AND @flag_1_ad = 'ASC'
      THEN CAST (a AS CHAR(n))
      WHEN @flag_1 = 'b' AND @flag_1_ad = 'ASC'
      THEN CAST (b AS CHAR(n))
      WHEN @flag_1 = 'c' AND @flag_1_ad = 'ASC'
      THEN CAST (c AS CHAR(n))
      ELSE NULL END AS sort_1_a,
CASE @flag_1
      WHEN @flag_1 = 'a' AND @flag_1_ad = 'DESC'
      THEN CAST (a AS CHAR(n))
      WHEN @flag_1 = 'b' AND @flag_1_ad = 'DESC'
      THEN CAST (b AS CHAR(n))
      WHEN @flag_1 = 'c' AND @flag_1_ad = 'DESC'
      THEN CAST (c AS CHAR(n))
      ELSE NULL END AS sort_1_d

.. ORDER BY sort_1_a ASC,  sort_1_d DESC

I have shown explicit CAST(<exp> AS CHAR(n)), but if the datatypes of
the THEN clause expressions were already the same, there would be no
reason to force the conversions.

You change the ELSE NULL clause to any constant of the appropriate
datatype, but it should be something useful to the reader.

A neater way of doing this is to use one column for each sorting option
so you do not have worry about CAST() operations.

SELECT ...
       CASE WHEN @flag = 'a' THEN a ELSE NULL END AS sort1,
       CASE WHEN @flag = 'b' THEN b ELSE NULL END AS sort2,
       CASE WHEN @flag = 'c' THEN c ELSE NULL END AS sort3
  FROM Foobar
 WHERE  ...
ORDER BY sort1, sort2, sort3;