执行 from emp 再执行 where deptno=20 后：
+-------+-------+---------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+---------+------+------------+---------+------+--------+
|  7369 | SMITH | CLERK   | 7902 | 1980-12-17 |  800.00 | NULL |     20 |
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+


执行group by 后，产生3个临时表：
+-------+-------+---------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+---------+------+------------+---------+------+--------+
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+

+-------+-------+-------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB   | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+-------+------+------------+---------+------+--------+
|  7369 | SMITH | CLERK | 7902 | 1980-12-17 |  800.00 | NULL |     20 |
|  7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
+-------+-------+-------+------+------------+---------+------+--------+

+-------+-------+---------+------+------------+---------+------+--------+
| EMPNO | ENAME | JOB     | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
+-------+-------+---------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+


执行select job,count(*)
读取第一张临时表的首行，并执行count(*)数出来第一张临时表有2行，
由此得到一个数据行
+---------+----------+
| job     | count(*) |
+---------+----------+
| ANALYST |        2 |
+---------+----------+

读取第二第三张临时表，得到
+-------+----------+
| job   | count(*) |
+-------+----------+
| CLERK |        2 |
+-------+----------+
和
+---------+----------+
| job     | count(*) |
+---------+----------+
| MANAGER |        1 |
+---------+----------+

最后把三个数据拼起来得到
+---------+----------+
| job     | count(*) |
+---------+----------+
| ANALYST |        2 |
| CLERK   |        2 |
| MANAGER |        1 |
+---------+----------+