SQL语句：用count求group by分组的个数

Question

如person表 pid,sid 两字段
我用sid分组，SQL语句如下：
select count(*) num,sid from person group by sid
结果如下：
num sid
5 001
10 002
12 003
0 004

我想要条SQL语句求共分有几组，如上结果是4组，应该怎么写？

Answer 1

1、创建测试表，create table test_group(pid number, sid varchar2(20));

2、插入测试数据，

insert into test_group values(1,'001');

insert into test_group values(2,'001');

insert into test_group values(2,'002');

insert into test_group values(2,'002');

insert into test_group values(2,'002');

insert into test_group values(3,'003');

insert into test_group values(3,'003');

insert into test_group values(null,'004');

3、查询表中记录，select t.*, rowid from test_group t;

4、编写sql，将记录分组后，记录组数，结果为4组，

select count(*) from (select count(*) num,sid from test_group group by sid)

Answer 2

select count(*) num,sid into #a from person group by sid
select count(*) from #a

或者
select count(*) from (select count(*) num,sid from person group by sid )

Answer 3

from括号后面要带重命名
select count(*) from (select count(*) num,sid from person group by sid ) rename

Answer 4

select count(*) from (select sid from person group by sid) s;