Answer Posted / purushottam

All the above suggested point is fine but addition on it is
when you are going to use the aggregate function for same
table then also be the mutating table error will occur.

example:

create table t(x numner);
/

create or replace trigger t_af_trigg
before insert into t
for each row
declare
n integer;
begin
select count(*) into n from t;
dbms_output.put_line('there are ' || n || ' rows in t');
end;
/

insert into t values(1);
error:
ORA-04091: table MISC.T is mutating, trigger/function may
not see it
ORA-06512: at "MISC.T_AF_TRIGG", line 5
ORA-04088: error during execution of trigger 'MISC.T_AF_TRIGG'

This example value addition of Vivek's suggession

"Mutating means that some one is trying to access the table
currently being held by some other non-committing
transaction(i.e. in Locked state)"

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