MySQL数据库-数据的查询操作大全_34.请查询 student 表中 age 字段的值小于 18 的数据-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_44850984/article/details/89918695

本文围绕MySQL数据库展开，详细介绍了各类查询操作。包括基本查询、条件查询、空判断、排序、聚合函数、分组、分页、链接查询、自关联以及子查询等。通过大量示例代码，展示了不同查询方式的具体用法，为数据库查询操作提供了全面的参考。

查询

基本查询

查询所有字段

select * from 表名;

select * from students;
```
select * from classes;
```
select id, name from classes;

查询指定字段

select 列1,列2,… from 表名;

select name, age from classes;

select 表名.字段 … from 表名;

select students.name, students.age from students;

使用 as 给字段起别名

select 字段 as 名字… from 表名;

select name as 姓名, age as 年龄 from classes;

可以通过 as 给表起别名

select 别名.字段 … from 表名 as 别名;

select students.name, students.age from students;
select s.name, s.age from students as s;
失败的select students.name, students.age from students as s;

消除重复行

distinct 字段

select distinct gender from students;

条件查询

elect … from 表名 where …

比较运算符

查询大于18岁的信息
select * from students where age>18;
select id,name,gender from students where age>18;

查询小于18岁的信息
select * from students where age<18;

>= <=

查询小于或等于18岁的信息

查询年龄为18岁的所有学生的名字
select * from students where age=18;

!= 或者 <>

逻辑运算符

and

18到28岁之间所有学生的信息
select * from students where age>18 and age<28;
失败select * from students where age>18 and <28;
18岁以上的女性
select * from students where age>18 and gender=“女”;
select * from students where age>18 and gender=2;

18岁以上或者身高查过180(包含)以上
select * from students where age>18 and height>=180;

not

不在 18岁以上的女性这个范围内的信息
select * from students where not age>18 and gender=2;
select * from students where not (age>18 and gender=2);
年龄不是小于或者等于18 并且是女性
select * from students where (not age<=18) and gender=2;

模糊查询

like
% 替换1个或者多个
_ 替换1个

查询姓名中以 “小” 开始的名字
select name from students where name=“小”;
select name from students where name like “小%”;
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 “__”;
查询有3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

查询以周开始的姓名
select name from students where name rlike “^周.*”;
查询已周开始、伦结尾的姓名
select name from students where name rlike “^周.*伦$”;

范围查询 in（1, 3，8 ）表示在一个非连续的范围内

查询年龄为18、34的名字
select name,age from students where age=18 or age=34;
select name,age from students where age=18 or age=34 or age=12;
select name,age from students where age in (12, 18, 34);

not in 不非连续的范围之内
年龄不是 18、34岁之间的信息
select name,age from students where age not in (12, 18, 34);

between … and …表示在一个连续的范围内

查询年龄在18到34之间的信息
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

查询年龄不在在18到34之间的的信息
select * from students where age not between 18 and 34;
select * from students where not age between 18 and 34;
失败的select * from students where age not (between 18 and 34);

空判断

判空is null

查询身高为空的信息
select * from students where height is null;
select * from students where height is NULL;
select * from students where height is Null;

判非空is not null

select * from students where height 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

查询年龄在18到34岁之间的男性，按照年龄从小到到排序
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;
查询年龄在18到34岁之间的女性，身高从高到矮排序
select * from students where (age between 18 and 34) and gender=2 order by height desc;

order by 多个字段

查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
select * from students where (age between 18 and 34) and gender=2 order by height desc, age asc;
查询年龄在18到34岁之间的女性，身高从高到矮排序, 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动大到小排序
select * from students where (age between 18 and 34) and gender=2 order by height desc, age asc, id desc;
按照年龄从小到大、身高从高到矮的排序
select * from students order by age asc, height desc;

聚合函数

总数 count

查询男性有多少人，女性有多少人
select * from students where gender=1;
select count(*) from students where gender=1;
select count(*) as 男性人数 from students where gender=1;
select count(*) as 女性人数 from students where gender=2;

最大值max

查询最大的年龄
select age from students;
select max(age) from students;
查询女性的最高身高
select max(height) from students where gender=2;

最小值min

求和sum

计算所有人的年龄总和
select sum(age) from students;

平均值avg

计算平均年龄
select avg(age) from students;
计算平均年龄 sum(age)/count(* )
select sum(age)/count(*) from students;

四舍五入 round(123.23 , 1) 保留1位小数

计算所有人的平均年龄，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;
select round(avg(height), 2) from students where gender=1;

分组 group by

group by

# 对原始数据筛选分组 在聚合函数的基础上分组，否则没有意义

按照性别分组, 查询所有的性别
select gender from students group by gender;
失败select * from students group by gender;
失败 select name from students group by gender;
计算每种性别中的人数
select gender,count(*) from students group by gender;
计算男性的人数
select gender,count(*) from students where gender=1 group by gender;

group_concat(…)

分组后显示group_concat()内的内容 ()内写什么，就连接显示什么

select gender, group_concat(name) from students where gender=1 group by gender;
select gender, group_concat(name, age, id) from students where gender=1 group by gender;
select gender, group_concat(name, “_”, age, " ", id) from students where gender=1 group by gender;

having

# 对结果进行筛选 对分组进行条件判断
注：where对原始表进行条件判断，having 对分组进行条件判断；where在分组前，having在分组后

查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
select gender, group_concat(name), avg(age) from students group by gender having avg(age)>30;
查询每种性别中的人数多于2个的信息
select gender, group_concat(name) from students group by gender having count(*)>2;

分页

limit start,count

限制查询出来的数据格式
select * from students where gender=1 limit 2;
查询前5个数据
select * from students limit 0,5;
查询id6-10（包含）的书序
select * from students limit 5,5;
每页显示2个，第1个页面
select * from students limit 0,2;
每页显示2个，第2个页面
select * from students limit 2,2;
每页显示2个，第3个页面
select * from students limit 4,2;
每页显示2个，第4个页面
select * from students limit 6,2; – -----> limit (第N页-1)*每页的个数, 每页的个数;
每页显示2个，显示第6页的信息, 按照年龄从小到大排序
select * from students order by age asc limit 10,2;
select * from students where gender=2 order by age desc limit 0,2;
失败select * from students limit 2*(6-1),2;
失败select * from students limit 10,2 order by age asc;
注：limit 需放在最后

链接查询(多表查询）

inner join … on

select * from 表A inner join 表B on 条件;
注： inner join 两个表同时存在才显示；inner join on 条件显示相同条件的信息

查询有能够对应班级的学生以及班级信息
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;
select students.name, classes.name from students inner join classes on students.cls_id=classes.id;
给数据表起名字
select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;
查询能够对应班级的学生以及班级信息-显示学生的所有信息, 只显示班级名称
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.xxx s.xxx from student as s inner join clssses as c on … order by …;
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;
当时同一个班级的时候, 按照学生的id进行从小到大排序
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;

left join

# 左表中特有的数据，右表中没有与之对应的数据，则结果中用null填充

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 xxx as s left join xxx as c on… where …
select … from xxx as s left join xxx as c on… having …
select s.*, c.name from students as s left join classes as c on s.cls_id=c.id where c.name is null;
select s.*, c.name from students as s left join classes as c on s.cls_id=c.id having c.name is null;

right join

# 将数据表名字互换位置，用left join完成

自关联

在这里插入图片描述

注：chcp 65008 Windows改变为utf8编码

省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/
查询所有省份
select * from areas where pid is null;
查询aid
select aid from areas where “山东省”;
查询出山东省有哪些市
select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle=“山东省”;
select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle=“山东省”;
查询出青岛市有哪些县城
select * from areas as city inner join areas as province on city.pid=province.aid where province.atitle=“青岛市”;

子查询

标量子查询

查询出高于平均身高的信息
select * from students where height > (select avg(height) from students);
查询最高的男生信息
select * from students where height = 188;

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