请教一个简单的问题？

[tab888]

if not lb_sign then
if trim(ls_sign_result_string) = '' then
ls_sign_result_string = fill('0',256)
else
ls_sign_result_addstring = fill('0',216)
ls_sign_result_string = ls_sign_result_string + ls_sign_result_addstring

end if

怎样才能判断if trim(ls_sign_result_string) = '' 不成功，就执行else语句
而不是if not lb_sign不成功，就执行else语句

[tab888]

if not lb_sign then
if trim(ls_sign_result_string) = '' then
ls_sign_result_string = fill('0',256)
else
ls_sign_result_addstring = fill('0',216)
ls_sign_result_string = ls_sign_result_string + ls_sign_result_addstring

end if

怎样才能让if trim(ls_sign_result_string) = '' 不成功，就执行else语句
而不是if not lb_sign不成功，就执行else语句

[workhand]

Return value
String. Returns a copy of string with all leading and trailing spaces deleted if it succeeds and the empty string ("") if an error occurs. If string is NULL, Trim returns NULL.

[TTLOVEYOU3344]

isnull()函數