Alias - Alias the AS keyword using a column name:
mysql> select sno as studentId,sname as studentName from student; +-----------+-------------+ | studentId | studentName | +-----------+-------------+† 12001 | bgg | | 96001 | Liu Jun| | 95002 | Liu Chen| | 94001 | Cottage| | 95005 | Zhang San| | 95004 | Zhang Li | | 95001 | Li Yong| | 97001 | Li Si| | 95003 | Wang Min| | 96004 | Hibiscus| +-----------+-------------+ 10 rows in set (0.10 sec) // Omitting the as keyword in mysql/mariadb can also achieve mysql> select sno studentId, sname studentName from student; +-----------+------------ -+ | studentId | studentName | +-----------+-------------+† 12001 | bgg | | 96001 | Liu Jun| | 95002 | Liu Chen| | 94001 | Cottage| | 95005 | Zhang San| | 95004 | Zhang Li | | 95001 | Li Yong| | 97001 | Li Si| | 95003 | Wang Min| | 96004 | Hibiscus| +-----------+-------------+ 10 rows in set (0.07 sec)
GROUP BY statement:
For example, when summing (sum function), it is often necessary to use the group by statement to perform classification and summation
//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 | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+ //find the total cost of sal for each job type 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) //Check the number of boys and girls in the student table mysql> select ssex,count(sage) from student group by ssex; +------+-------------+ | ssex | count(sage) | +------+-------------+ | M | 1 | | Female | 3 | | Male | 6 | +------+-------------+
2018-05-01 23:21:52