mysql查询语句

文章目录

mysql查询语句
- 准备数据
- 查询
- 条件查询
- 逻辑运算符
- 模糊查询
- 范围查询
- 空判断
- 排序
- 聚合函数
- 分组
- 分页
- 连接查询
- 自关联
- 子查询

准备数据

准备一个数据库
两个表students，classes
准备数据students

insert into students values
(0,'小明',18,180.00,2,1,0),
(0,'小月月',18,180.00,2,2,1),
(0,'彭于晏',29,185.00,1,1,0),
(0,'刘德华',59,175.00,1,2,1),
(0,'黄蓉',38,160.00,2,1,0),
(0,'凤姐',28,150.00,4,2,1),
(0,'王祖贤',18,172.00,2,1,1),
(0,'周杰伦',36,null,1,1,0),
(0,'程坤',27,181.00,1,2,0),
(0,'刘亦菲',25,166.00,2,2,0),
(0,'金星',33,162.00,3,3,1),
(0,'静香',12,180.00,2,4,0),
(0,'郭靖',12,170.00,1,4,0),
(0,'周杰',34,176.00,2,5,0);

准备数据classes

insert into classes values(0,"python_01期"),(0,"python_02期"),(0,"python_04期");

查询

查询所有字段

select * from 表名;
select * from students;
select * from classes;

一定条件查询

select * from 表名 where 条件;
select * from student where name="老王";
select * from student where id=3;

查询指定字段

select 字段1,字段2... from 表名;
select name,age from students;

可以使用as为字段指定别名

select 字段1 as 别名1,字段2 as 别名2... from 表名;
select name as 姓名,ager as 年龄 from students;
select gender as 性别,name as 姓名 from students;

可以使用as为表指定别名

select 表别名.字段1,表别名.字段2... from 表名  as 表别名;
select sss.name,sss.age from students as sss;
select student.name,student.age from students as sss;(失败);

消除重复行

select distinct 字段 from 表名;
select distinct gender from students;

条件查询

select ... from 表名 where ....

比较运算符

查询大于18岁的信息

select * from students where age>18;

查询小于18岁的信息

select * from students where age<18;

<=
>=

查询小于等于/大于等于18岁的信息

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

查询等于18岁的所有学生的名字

select name from students where age=18;

!=

查询不等于18岁的所有学生的名字

select name from students where age!=18;

逻辑运算符

and

18到28之间的所以学生信息

select * from students where age>18 and age<28;
select * from students where age＞18 and ＜28（这样会报错）

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;

or

18以上或者身高超过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); -- (正确)

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个或多个
- _替换1个（必须有一个）

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”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.*";

查询以周开始，伦结尾的姓名

select name from students where name rlike "^周.*伦$";

范围查询

in

in(1，3，8)表示在一个非连续的范围内

查询年龄为18、34的姓名

select name,age from students where age in (12,18,34);

not in不非连续性的范围之内

查询年龄不是18、34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;
// 下面是失败的例子
select * from students where age not (between 18 and 34);
select * from students where not age 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;

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，按照年从小到大排序

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;//从小到大
select * from students where (age between 18 and 34) and gender=1 order by age desc;//从大到小

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从大到小排序

select * from students where (age between 18 and 34) and gender=2 order by height desc,age asc,id desc;

按照年先按龄从小到大、再同时按身高从高到矮的排序

select * from students order by age asc,height desc;

聚合函数

count总数

查询男性有多少人，女性有多少人

select count(*) as 男性人数 from students where gender=1;
select count(*) as 女性人数 from students where gender=2;

max最大值
min最小值

查询最大/最小的年龄

select age from students ;
select max(age) as 最大年龄 from students ;
select min(age) as 最大年龄 from students ;

查询女性的最高/最矮身高

select max(height) as 最高身高 from students where gender=2;
select min(height) as 最高身高 from students where gender=2;

sum求和

计算所有人的年龄总和

select sum(age) from students ;

avg平均值

计算所有人的平均年龄

select avg(age) from students ;

计算平均年龄 sum(age)/count(*)

select sum(age)/count(*) from students ;

round(123.23,1)四舍五入，保留一位小数

计算所有人的平均年龄，保留两位小数

select round(sum(age)/count(*),2) from students;

计算男性的平均身高，保留两位小数

select round(avg(height),2) from students where gender=1;

分组

分组(配合聚合函数使用)

group by

按照性别分组，查询所有的性别

select gender from students group by gender;

计算每种性别的信息

select gender,count(*) from students group by gender;
select gender,avg(age) from students group by gender;
select gender,max(age) 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(...)

查询同种性别中的姓名

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

having 查出结果进行判断

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的信息

select gender,group_concat(name) from students group by gender having count(*)>2;

分页

limit start(开始位置)，count(总数)

限制查询出来的数据个数

select * from students where gender=1 limit 2;

查询前5个数据

select * from students where gender=1 limit 开始位置,数据个数;
select * from students where gender=1 limit 0,5;
select * from students where gender=1 limit 5,5;
select * from students where gender=1 limit 10,5;

查询id6-10（包含）的顺序

select * from students where gender=1 limit 5,5;

每页显示2个，第1个页面

select * from students where gender=1 limit 0,2;

每页显示2个，第2个页面

select * from students where gender=1 limit 2,2;

每页显示2个，第3个页面

select * from students where gender=1 limit 4,2;

每页显示2个，第4个页面

select * from students where gender=1 limit 6,2;

*limit (第N页-1)每页个数，每页个数；

每页显示2个，显示第6个页面的信息，按照年龄从小到大排序

limit 在最后

（失败）select *from students limit 2*(6-1),2;
（失败）select *from students limit 10,2 order by age asc;
select * from student order by age asc limit 10,2;

查询所有女性的身高信息，从高到矮排序，只显示两个

select * from students where gender=2 order by height desc limit 0,2；

连接查询

inner join ... on

select … from 表A inner join 表B;

select * from students inner join classes;

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;

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;

查询有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;

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,id;

left 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

从原表里面找用where，从结果表中找用having

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

right join

将数据表名字互换位置，用left join完成

自关联

一张表的时候

select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle="xx省";

子查询

selcet语句里面嵌套一个子select，先执行子select

查询最高的男生信息

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