Константин

update teacher t
   set hiredate = (select min(lessondatetime)
                     from lesson l
                    where t.pc_teacher = l.teacherpc)

insert into tb
select case when cond1 then 1 else null num,
       case when not cond1 then 'a' else null str
from ...

create table test_a(num number, str varchar2(100));

insert into test_a
with
  generator as
   (select case
             when dbms_random.value() > 0.5 then dbms_random.string('p', 10)
             else to_char(trunc(dbms_random.value(0, 100)))
           end str
      from dual
      connect by level <= 50),
  classificator as
   (select str, case
                  when translate(str, chr(10)||'0123456789', chr(10)) is null then 'number'
                  else 'string'
                end type
      from generator)
select case
         when type = 'number' then to_number(str)
         else null
       end num,
       case
         when type = 'string' then str
         else null
       end str
  from classificator;
commit;

select * from test_a;