Why this update is not working , can't I use nvl function in update st

From: Veera Prasad <vprasad_at_olf.com>
Date: Thu, 26 Oct 2000 13:47:19 -0400
Message-Id: <10661.120313@fatcity.com>

Hi Guys,
Can anybody tell me why this update is failing , can't I use nvl function in update while
loop thru?, please also see the errors at the end.

DECLARE

v_min_unique_num int;
v_max_unique_num int;
v_increment int;

begin

   v_increment := 10000;
v_min_unique_num = 1;
v_max_unique_num = 100;
 while (v_min_unique_num <= v_max_unique_num) loop update <table1>
set max_capacity = nvl( (select max (col1) from <table2>

   where <table2>.col1 = <table1.col1 and     <table1.col1 in (select col1 from <table3>       where col2 = 33)), 0.0)
where <table1>.col1 between v_min_unique_num and (v_min_unique_num + v_increment);
  v_min_unique_num := v_min_unique_num + v_increment + 1;  end loop;
end;
/

PLS-00103: Encountered the symbol "SELECT" when expecting one of the following:
( - + mod not null others <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance execute forall time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string>

ORA-06550: line 78, column 31:
PLS-00103: Encountered the symbol "," when expecting one of the
following:

; return returning and or Received on Thu Oct 26 2000 - 12:47:19 CDT