oracle group by與分組列為null空

SQL> create table t_group(a int,b int);

Table created

SQL> insert into t_group values(null,1);

1 row inserted

SQL> insert into t_group values(null,2);

1 row inserted

SQL> insert into t_group values(1,2);

1 row inserted

SQL> insert into t_group values(3,2);

1 row inserted

SQL> commit;

Commit complete

SQL> select * from t_group;

                                      A                                       B
--------------------------------------- ---------------------------------------
                                                                              1
                                                                              2
                                      1                                       2
                                      3                                       2

SQL> select a,count(b) from t_group  group by a;

                                      A   COUNT(B)
--------------------------------------- ----------
                                                 2
                                      1          1
                                      3          1