confusion in except and not in keywords

Hii,

According to the definition, EXCEPT operator compares values in one or more than one column. This means that the number of columns must be the same. On the other hand, the NOT IN operator compares values in a single column.

 

So In your case, this one "select FIR_REG_NUM from t_declare_reward where FIR_REG_NUM not in(Select FIR_REG_NUM from t_arrest_memo)" only one column is checked and if the value does not match it returns nothing.

Similarly, for the other case, while using "EXCEPT" it checks in all respective columns.

 

That's the difference, Hope this makes sense.