Outer Join puzzle - How this works?

Hi All,

I have the two following queries alongwith the results. Can anybody interpret the results (fourth sql) and let me know why it works like this
(fourth sql).

SQL> select * from a;

           NUM
-        - - - - - - 
                       1
                       2
                       3

        SQL>  select * from b;
                   NUM1        NUM2
-        - - - - - -         - - - - -
             1                                      << NUM2 is null in this
record
2       2
3       1

SQL>  select  NUM, NUM1, NUM2  from a, b
           Where nvl(num2,1) = 1 and num=num1(+);
           NUM1        NUM2         NUM3
-        - - - - - -         - - - - -        - - - - - -
1       1
                3                  3                1

SQL>  select  NUM, NUM1, NUM2  from a, b
           Where nvl(num2(+),1) = 1 and num=num1(+);
           NUM1        NUM2         NUM3
-        - - - - - -         - - - - -        - - - - - -
1       1
2        
3       3                 1

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