mysql> select sno as studentId,sname as studentName from student;+-----------+-------------+| studentId | studentName |+-----------+-------------+| 12001 | bgg || 96001 | 刘军 || 95002 | 刘晨 || 94001 | 山寨 || 95005 | 张三 || 95004 | 张立 || 95001 | 李勇 || 97001 | 李四 || 95003 | 王敏 || 96004 | 芙蓉 |+-----------+-------------+10 rows in set (0.10 sec)//在mysql/mariadb中省略as 关键字也可实现mysql> select sno studentId,sname studentName from student;+-----------+-------------+| studentId | studentName |+-----------+-------------+| 12001 | bgg || 96001 | 刘军 || 95002 | 刘晨 || 94001 | 山寨 || 95005 | 张三 || 95004 | 张立 || 95001 | 李勇 || 97001 | 李四 || 95003 | 王敏 || 96004 | 芙蓉 |+-----------+-------------+10 rows in set (0.07 sec)
比如在合计（sum函数）的时候常需要用group by语句来进行分类求和
//emp表格中的数据mysql> select * from emp;+-------+--------+-----------+------+------------+---------+---------+--------+| empno | ename | job | mgr | hiredate | sal | comm | deptno |+-------+--------+-----------+------+------------+---------+---------+--------+| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 || 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 || 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 || 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 || 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 || 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 || 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 || 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 || 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 || 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 || 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 || 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 || 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 || 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |+-------+--------+-----------+------+------------+---------+---------+--------+//查找每个job种类一共支出的sal是多少mysql> select job,sum(sal) from emp group by job;+-----------+----------+| job | sum(sal) |+-----------+----------+| ANALYST | 6000.00 || CLERK | 4150.00 || MANAGER | 8275.00 || PRESIDENT | 5000.00 || SALESMAN | 5600.00 |+-----------+----------+5 rows in set (0.12 sec)//查看student表中的男女生人数mysql> select ssex,count(sage) from student group by ssex;+------+-------------+| ssex | count(sage) |+------+-------------+| M | 1 || 女 | 3 || 男 | 6 |+------+-------------+
2018-05-01 23:21:52
