oracle 不支持 =和!=

-- Created on 2012/11/20 by JWW 
declare
  -- Local variables here
  i  integer := 2;
  j  integer := 2;
  k  varchar2(60);
  op varchar2(45);
begin
  -- Test statements here
  k := '';
  if (i = j and k = '') then
    --能不能输出
    op := 'k = ""有结果';
  else
    op := 'k is null有结果';
  end if;
end;

k 无论赋值还是不赋值，赋'' 或null ,都是 k is null 才生效，= ，!=无效