编辑代码

-- DQL 查询语句中的聚合函数;
create database info;
use info;
create table worker(
    id int,
    name varchar(10),
    age int,
    gender char(1),
    workcity varchar(10),
    entrytime date
);
insert into worker values (1,'张三',28,'F','上海','2020-08-20');
insert into worker values(2,'李四',24,'F','上海','2021-06-24');
insert into worker values(3,'王二麻',27,'M','北京','2020-07-22');
insert into worker values(4,'王五',29,'M','深圳','2022-01-10');
insert into worker values(5,'赵六',32,'F','深圳','2019-07-07');
insert into worker values(6,'钱九',31,'M','北京','2018-04-19');
insert into worker values(7,'孙八',30,'F','深圳','2021-09-17');
insert into worker values(8,'何十',34,'M','北京','2022-03-28');
insert into worker values(9,'九九',26,'M','深圳','2023-08-19');
insert into worker values(10,'零七',24,'F','上海','2021-06-24');
insert into worker values(11,'可可',27,'M',null,'2024-07-21');
-- select * from worker; 
-- SQL 语句中的聚合函数,是对一列数据进行纵向/列计算;
-- 常见聚合函数 count,sum,min,max,avg;(统计整张表的数据 求和 最小 最大 平均值)
-- 常用的语法是 select 聚合函数(字段)from 表名; 所有的聚合函数不计算null的值
select count(*) from worker;
select count(id) from worker;
select avg(age) from worker;
select max(age) from worker;
select min(age) from worker;
select sum(age) from worker where workcity='深圳';