alias--uses a column name alias as keyword:
Mysql>SelectSno asStudentid,sname asStudentname fromstudent;+-----------+-------------+| StudentID | Studentname |+-----------+-------------+|12001| BGG | |96001| Liu June | |95002| Liu Chen | |94001| Cottage | |95005| Zhang San | |95004| Zhang Li | |95001| Li Yong | |97001| John Doe | |95003| Wang min | |96004| Hibiscus |+-----------+-------------+TenRowsinch Set(0.10sec)//omitting the AS keyword in MYSQL/MARIADB can also be implementedMysql>SelectSno Studentid,sname Studentname fromstudent;+-----------+-------------+| StudentID | Studentname |+-----------+-------------+|12001| BGG | |96001| Liu June | |95002| Liu Chen | |94001| Cottage | |95005| Zhang San | |95004| Zhang Li | |95001| Li Yong | |97001| John Doe | |95003| Wang min | |96004| Hibiscus |+-----------+-------------+TenRowsinch Set(0.07Sec
GROUP by statement:
For example, in the aggregate (SUM function) often need to use the group by statement to classify and sum
The data in the EMP table 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 | |+-------+--------+-----------+------+------------+---------+---------+--------+//find each job type Total cost of Sal is how much MySQL > select Job,sum (SAL) from the 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)//view student table for male and female students mysql> Select Ssex,count (Sage) from Student GROUP BY ssex;+------+-------------+| Ssex | Count (Sage) |+------+-------------+| M | 1 | | Women | 3 | | Male | 6 |+------+-------------+
2018-05-01 23:21:52
MYSQL/MARIADB study Record--Query 2