Mutating table confusion

Question

I am confused. I thought that if you tried to query a table from a BEFORE ROW trigger, which was the actual table upon which the BEFORE ROW trigger was applied against, you would get a MUTATING TABLE ERROR. For instance, I did not think that this would work:
SQL> create table check_mte (tk number,
  2                          group_id number,
  3                          date_begin date,
  4                          date_end date);
Table created.
SQL> create or replace
  2  trigger check_mte
  3  before insert
  4  on check_mte
  5  for each row
  6  declare
  7    wk_cnt number;
  8  begin
  9    select count(*) into wk_cnt
10      from check_mte a
11     where a.tk != :new.tk
12       and a.group_id = :new.group_id
13       and (:new.date_begin, :new.date_end) overlaps (a.date_begin, a.date_end);
14   
15    if wk_cnt = 1 then
16      raise_application_error(-20500, 'Overlapping date range error.');
17    end if;
18   
19  end check_mte;
20  /
Trigger created.
SQL> insert into check_mte values (1, 1, trunc(sysdate), trunc(sysdate)+1);
1 row created.
SQL> insert into check_mte values (2, 1, trunc(sysdate)+1, trunc(sysdate)+2);
1 row created.
SQL> insert into check_mte values (3, 1, trunc(sysdate)+2, trunc(sysdate)+3);
1 row created.
SQL> select * from check_mte;
        TK   GROUP_ID DATE_BEGI DATE_END
         1          1 25-OCT-10 26-OCT-10
         2          1 26-OCT-10 27-OCT-10
         3          1 27-OCT-10 28-OCT-10
SQL> insert into check_mte values (4, 2, trunc(sysdate), trunc(sysdate)+1);
1 row created.
SQL> insert into check_mte values (5, 2, trunc(sysdate), trunc(sysdate)+1);
insert into check_mte values (5, 2, trunc(sysdate), trunc(sysdate)+1)
ERROR at line 1:
ORA-20500: Overlapping date range error.
ORA-06512: at "CHECK_MTE", line 11
ORA-04088: error during execution of trigger 'CHECK_MTE'Why is this not generating a Mutating Table Error?
--=Chuck

Answer 1

It's a multi-row update that causes the issue, not a single row update:
SQL> insert into check_mte select * from check_mte;
insert into check_mte select * from check_mte
ERROR at line 1:
ORA-04091: table CHECK_MTE is mutating, trigger/function may not see it
ORA-06512: at "CHECK_MTE", line 4
ORA-04088: error during execution of trigger CHECK_MTE'