mysql/mariadb学习记录——查询3(AVG、SUM、COUNT）函数
AVG（） 求平均数函数：
//求emp表中的sal属性的平均值 mysql> select avg(sal) as salAverage from emp; +-------------+ | salAverage | +-------------+ | 2073.214286 | +-------------+ 1 row in set (0.11 sec) //emp表数据 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 | +-------+--------+-----------+------+------------+---------+---------+--------+
SUM（）求和函数：
mysql> select sum(sal) as salSum from emp; +----------+ | salSum | +----------+ | 29025.00 | +----------+ 1 row in set (0.11 sec)
count（）计数函数：
//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 | +------+-------------+
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