2.数据库查询

Posted 2022-10-11 wjzheng

-- 数据的准备

    -- 创建一个数据库

    create database python_test charset=utf8;

 

    -- 使用一个数据库

    use python_test;

 

    -- 显示使用的当前数据是哪个?

    select database();

 

    -- 创建一个数据表

    -- students表

    create table students(

        id int unsigned primary key auto_increment not null,

        name varchar(20) default ‘‘,

        age tinyint unsigned default 0,   # tinyin 最小值

        height decimal(5,2),  # decimal 小数

        gender enum(‘男‘,‘女‘,‘中性‘,‘保密‘) default ‘保密‘,

        cls_id int unsigned default 0,

        is_delete bit default 0

    );

 

    -- classes表

    create table classes (

        id int unsigned auto_increment primary key not null,

        name varchar(30) not null

    );

 

准备数据

-- 向students表中插入数据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);

 

-- 向classes表中插入数据insert into classes values (0, "python_01期"), (0, "python_02期");

 

 

-- 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 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 students;

 

    -- select 表名.字段 .... from 表名;

    select students.name, students.age from students;

 

    

    -- 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

    -- 比较运算符

        -- select .... from 表名 where .....

        -- >

        -- 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;

 

        -- >=

        -- <=

        -- 查询小于或者等于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之间的所以学生信息

        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;

 

 

        -- or

        -- 18以上或者身高查过180(包含)以上

        select * from students where age>18 or 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 "__";   # 两个下划线

 

        -- 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.*";  # ^ 表示开头

 

        -- 查询以 周开始、伦结尾的姓名

        select name from students where name rlike "^周.*伦$";  # $ 表示结尾

 

 

    -- 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的姓名

        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;    # not between 是个整体

        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;  # 默认asc

    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,id desc;

 

 

    -- 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;

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

 

-- 分组（一般和聚合一起使用）

 

    -- group by

    -- 按照性别分组,查询所有的性别

     -- select name from students group by gender;

     -- select * from students group by gender;

    select gender from students group by gender;

    -- 失败select * 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(...)

    -- 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, 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

    -- 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;

 

     -- where和having的区别：

     -- where对元素表进行条件判断，having对查询的结果的条件判断

 

 

-- 分页

    -- limit start, count

 

    -- 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 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)*每个的个数, 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

    -- 失败select * from students limit 2*(6-1),2;

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

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

 

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

 

 

 

-- 连接查询（多表情况下）

    -- inner join ... on  # 内连接

 

    -- 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;

    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 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  on

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

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

 

    -- right join  on

    -- 将数据表名字互换位置，用left join完成

 

-- 自关联

    -- 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 province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle="青岛市";

    select * from areas where pid=(select aid from areas where atitle="青岛市")

 

 

-- 子查询（一个select中嵌套这另一个select）

    -- 标量子查询

    -- 查询出高于平均身高的信息

 

    -- 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);

 

    -- 列级子查询

    -- 查询学生的班级号能够对应的学生信息

    -- select * from students where cls_id in (select id from classes);