confusion in except and not in keywords

Question

Hello All,

 

I am using 2 queries:

 

1.  

select FIR_REG_NUM from t_declare_reward where FIR_REG_NUM not in(Select FIR_REG_NUM from  t_arrest_memo)

 

2.
select FIR_REG_NUM from t_declare_reward
except
select FIR_REG_NUM from t_arrest_memo 

 

2nd query returns  some rows, and its right. but I don't understand , why 1st query returns 0 rows.

Output should be come same from both queries.But 1st query gives wrong output.

 

Please clear.

 

Thanks in advance.