SQL & PL/SQL

Hi friends...

i executes following thing on emp table -

Update emp
set hire_date=sysdate;

Execute successfully with all rows updated with sysdate...

Now..
select hire_date from emp
where hire_date=sysdate;

Result:-No row selected..

and if i put in where clause :- "Where hire_date LIKE sysdate;"

It executes successfully...

I want to know the reason of this behavior...that previously when i was using in update command "hire_date=sysdate" it works...but not while selecting "hire_date=sysdate"...

Can anyone please clear this point...and yes about the "LIKE sysdate" also...that why it works...