order 排序 与 聚合函数
排序查询语法:
select * from 表名 order by 列1 asc/desc 【列2 asc/desc】
说明:1.将行数据按照列1进行排序,如果某些行 列1的值相同时,就按照列2 排序 , 以此类推
2.asc : 从小到大排序
3. desc : 从大到小排序
4. 默认按照列值从小到大排列
(例如 : select *from students where age between 18 and 34 order by height desc , id desc;)
常见的聚合函数:
count(*):统计数量 (select count(*) from student where gender = "女");
max(字段):找出该字段的最大值 (select max(age) from student where gender = "女");
min (字段):找出该字段的最小值 (select min(age) from student where gender = "女");
sum(字段):对该字段 求和 (select sum(age) from student where gender = "女");
avg (字段):对该字段求平均值 (select avg(age) from student where gender = "女");
四舍五入函数:round(数值,保留的小数位数)