MySQL 查询（条件、分组、聚合、分页、连接、子查询）

MySQL查询 - as & distinct

-- 使用 as 给字段起别名
-- select 字段 as 字段别名 from 表名;
select name as 姓名, age as 年龄 from students;
-- 使用 as 给表起别名
-- select 表别名.字段 from 表名 as 表别名;
select students.name, students.age from students;
select s.name, s.age from students as s;
-- 使用 distinct 消除重复行
select distinct gender from students;

一、条件查询

1. 比较运算符
   • >, <=, =, !=

select * from students where age>18;
select * from students where age=18;

2. 逻辑运算符
   • and & or & not: 与或非

-- and & or
select * from students where age>18 and age<28;
select * from students where age>18 or height>=180;
-- not
select * from students where not (age>18 and gender=2);
select * from students where (not age<=18) and gender=2;

3. 模糊查询
   • like: % 匹配1个或者多个，_ 匹配1个
   • rlike: 正则匹配

-- like 
-- 
-- 查询姓名中 有 "小" 所有的名字
select name from students where name like "%小%";
-- 查询有2个字的名字
select name from students where name like "__";
-- 查询至少有2个字的名字
select name from students where name like "__%";
-- rlike 正则
select name from students where name rlike "^周.*伦$";

4. 范围查询
   • in (a, b): 集合匹配
   • between and: 范围匹配

-- in (1, 3, 8) 非连续范围
select name,age from students where age in (12, 18, 34);
-- not in 非连续范围
select name,age from students where age not in (12, 18, 34);
-- between and 连续范围
select name, age from students where age between 18 and 34;
-- not between and 表示不在一个连续的范围内
select * from students where age not between 18 and 34;
select * from students where not age between 18 and 34;
-- 错误：select * from students where age not (between 18 and 34);

5. 空判断
   • is null & is not null: 空 & 非空

-- null 或 Null 或 NULL 都行
select * from students where height is null
select * from students where height is not null;

二、排序 - order by [asc/desc]

-- select 默认以主键从小到大排序
-- order by 字段 [asc/desc]
select * from students where (age between 18 and 34) and gender=1;
select * from students where (age between 18 and 34) and gender=1 order by age;
select * from students where (age between 18 and 34) and gender=1 order by age asc;
-- order by 多个字段
-- 如果身高相同的情况下按照年龄从小到大排序，如果年龄也相同那么按照id从大到小排序
select * from students where gender=2 order by height desc,age asc,id desc;

三、分组与聚合函数

聚合函数 - count()/max()/min()/sum()/avg()

-- 总数-count()
select count(*) as 男性人数 from students where gender=1;
-- 最大最小值-max()/min()
select max(age) from students;
-- 求和-sum
select sum(age) from students;
-- 平均值-avg
select avg(age) from students;

• 综合使用：

-- 计算平均年龄 sum(age)/count(*)
select sum(age)/count(*) from students;
-- 四舍五入-round(x,n) 四舍五入x保留n位小数
-- 计算平均年龄，保留2位小数
select round(sum(age)/count(*), 2) from students;
select round(sum(age)/count(*), 3) from students;
-- 计算男性的平均身高 保留2位小数
select round(avg(height), 2) from students where gender=1;
-- 错误：select name, round(avg(height), 2) from students where gender=1;

分组 - group by [having]

• 分组和聚合函数一起用，语法：

select 字段a, 聚合函数()
from 表名
where 字段n 符合 条件
group by 字段a;

• 使用举例：

-- 显示一列所有的性别
select gender from students group by gender;
-- 计算男性的人数
select gender,count(*) from students where gender=1 group by gender;
-- 错误：select name from students group by gender;
-- 错误：select * from students group by gender;

• group_concat()

-- 姓名按性别分组
select gender,group_concat(name) from students group by gender;
select gender,group_concat(name, " ", age, " ", id) from students where gender=1 group by gender;

• having
  • where和having区别：where 是筛选数据来分组，having 是对分组后的数据进行筛选

-- 查询平均年龄超过30岁的姓名，并按性别分组
select gender, group_concat(name),avg(age) from students group by gender having avg(age)>30;
-- 查询人数多于2个的性别分组
select gender, group_concat(name) from students group by gender having count(*)>2;

四、分页 - limit

• 语法：limit 开始, 个数

-- 查询前5个数据
select * from students limit 5;
select * from students limit 0, 5;
-- 查询从第6个开始之后的总共10个数据(6-15)
select * from students limit 5, 10;
-- 不支持运算符，错误：select * from students limit 2*(6-1),2;

• limit 必须要写在 sql 语句的最后

-- 错误：select * from students limit 10,2 order by age asc;
select * from students order by age asc limit 10,2;

五、连接查询

• 内连接：inner join on
  • 若没有写 on 条件，classes 表的内容将顺序重复连接到 students 表

select * from students inner join classes on students.cls_id=classes.id;

-- 按照要求显示姓名、班级
select students.*, classes.name from students inner join classes on students.cls_id=classes.id;
-- 查询有能够对应班级的学生的所有信息，并显示班级名称，并给数据表起别名
select s.*, c.name from students as s inner join classes as c on s.cls_id=c.id;
-- 在以上的查询中，将班级姓名显示在第1列
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id;
-- 查询有能够对应班级的学生以及班级信息, 按照班级进行排序
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

• 外连接：left join (& right join)

-- 查询每位学生对应的班级信息
select * from students as s left join classes as c on s.cls_id=c.id;
-- 查询没有对应班级信息的学生
select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;

自关联&子查询

1. 自关联

-- 省份和城市的信息都存在同一张表，查山东省有哪些市
select province.name, city.name 
from areas_tb as province 
inner join areas_tb as city 
on city.pid=province.id 
having province.name="山东省";
-- 上面等价于下面这条子查询语句
select * from areas_tb where pid=(select id from areas_tb where name="山东省")

2. 子查询

select * from students where height = (select max(height) from students);