--查询基本使用(条件，排序，聚合函数，分组，分页)
--创建学生表
create table students (
id int unsigned not null auto_increment primary key,
name varchar(20) default '',
age tinyint unsigned default 0,
high decimal(5,2),
gender enum('男', '女', '中性', '保密') default '保密',
cls_id int unsigned default 0,
is_delete bit default 0
);
--创建班级表
create table classes(
id int unsigned auto_increment primary key not null,
name varchar(20) not null
);
--往students表里插入数据
insert into students values
(0,'小明',18,180.00,1,1,0),
(0,'小月月',19,180.00,1,2,0),
(0,'彭于晏',28,185.00,1,1,0),
(0,'刘德华',58,175.00,1,2,0),
(0,'黄蓉',108,160.00,2,1,0),
(0,'凤姐',44,150.00,4,2,1),
(0,'王祖贤',52,170.00,2,1,1),
(0,'周杰伦儿',34,null,1,1,0),
(0,'程坤',44,181.00,1,2,0),
(0,'和珅',55,166.00,1,2,0),
(0,'刘亦菲',29,162.00,2,3,0),
(0,'金星',45,180.00,3,4,0),
(0,'静香',18,170.00,2,4,0),
(0,'郭靖',22,167.00,1,5,0),
(0,'周杰',33,178.00,1,1,0),
(0,'钱小豪',56,178.00,1,1,0),
(0,'谢霆锋',38,175.00,1,1,0),
(0,'陈冠希',38,175.00,1,1,0);
source
--查询
-- 查询所有列
--select * from 表名
select * from students;
--一定条件查询(where)
select * from 表名 where id=5;
select * from students where id=15;
-- 查询制定列
select id,name from students;
-- 使用as给字段起别名
select id,name as '姓名', age, high, gender from students;
select name as '姓名' , age as '年纪' from students;
-- 通过表名字段查询
select students.name from students;
-- 给表起别名查询
select s.id,s.name,s.age from students as s;
--消除重复行
-- distinct
select distinct age from students;
--条件查询
--比较运算符
-- 查询年纪大于18岁的信息
select * from students where age > 18;
--18岁到28岁之间(and)
select * from students where age >= 18 and age =< 28;
select * from students where age > 18 && age <=30;
select * from students where age between 18 and 28
-在18岁以上或者身高180以上的人(or)
select * from students where age > 18 or high > 180;
select * from students where age >20 || high >180;
-- 模糊查询
-- like
-- % 替代1个或者多个甚至是没有
-- 查询姓名中有‘小’的所有名字
select * from students where name like '%小%';
-- 查询两个字人的名字
下划线代表代表一个的意思
select * from students where name like '__';
-- 查询至少有2个字的名字
select * from students where name like '%__%';
--范围查询
-- in (1,3,8)表示在一个非连续的范围内
-- 查询 年纪为18和34的人
select * from students where age in (18, 34);
--查询 年龄在17岁到34岁之间的信息
select * from students where age between 17 and 34;
--查询 年纪不在18到34岁的信息
select * from students where age not between 17 and 34;
-- 空判断
-- 判断is null
-- 查询身高为空的信息
select * from students where high is null;
-- 判断非空is not null
select * from students where high is not null;
-- 排序
-- order by 字段
-- asc从小到大排列,即升序
-- desc从大到小排序，即降序
-- 查询年纪在18到34岁之间的男性，按照年纪从小到大
select * from students where gender=1 and age between 18 and 34 order by age;
-- 查询年纪在18到34岁之间的女性，身高从高到矮
select * from students where gender=2 and age between 18 and 34 order by high desc;
-- 查询年纪在18到34岁的女性，身高从高到矮排序，如果身高相同的情况下按照年纪从小到大排序
select * from students where age between 18 and 34 and gender=2 order by high desc;
-- 查询年纪在18到34岁的男性，身高从高到矮排序，如果身高相同的情况下按照年纪从小到大排序，如果年龄也相等那么按照id从小到大排序；
select * from students where age between 18 and 34 and gender=1 order by high desc, age, id desc;
--聚合函数
-- 总数
-- count
-- 查询男性有多少人
select count(*) from students where gender=1;
-- 最大值
-- max
-- 查询最大的年纪
select max(age) from students;
-- 查询女性的最高 身高
select max(high) from students where gender=2;
-- 最小值
-- min
select min(high) from students;
-- 求和
-- sum
-- 计算所有人的年龄总和
select sum(age) from students;
-- 平均值
-- avg
-- 计算平均年纪
-- 计算平均年纪 sum(age)/count(*)
select sum(age)/count(*) from students;
select avg(age),2 from students;
-- 保留2位小数
select round(avg(age),2) from students;
整数
-- 分组
-- group by
-- 按照性别分组，查询所有的性别
select gender from students group by gender;
+--------+
| gender |
+--------+
| 男 |
| 女 |
| 保密 |
| 中性 |
+--------+
-- 计算每组性别的人数
select gender, count(*) from students group by gender;
| gender | count(*) |
+--------+----------+
| 男 | 20 |
| 女 | 4 |
| 保密 | 1 |
| 中性 | 1 |
-- 查询男性组中的姓名 group_concat
select gender,group_concat(name,',',age ,'|') from students where gender=2 group by gender;
| 女 | 静香,18 | ,刘亦菲,29 | ,王祖贤,52 | ,黄蓉,108 |
-- having
-- 查询每个性别平均年纪超过30岁的性别，以及姓名 having avg(age) > 30
select gender, group_concat(name) from students group by gender having avg(age) > 30;
select gender, avg(age) from students group by gender having avg(age)>10;
+--------+----------+
| gender | avg(age) |
+--------+----------+
| 男 | 31.7000 |
| 女 | 51.7500 |
| 保密 | 45.0000 |
| 中性 | 44.0000 |
-- 查询每种性别中的人数多于4个的组的信息
select gender,group_concat(name) from students group by gender having count(*)>4;
-- 分页
-- 显示5页limit
select * from students limit 5;
-- 分页显示，每页显示2条数据
select * from students limit 0, 2;
-- 按照身高从高到矮排序，查找出所有女性，并且分页显示，每页显示2条数据
select * from students where gender=2 order by high desc limit 0,2;