数据的准备

创建一个数据库

create database python_test_1 charset=utf8;

使用一个数据库
```
use python_test_1;
```
显示使用的当前数据库是哪个
```
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,height 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
);
-- classes表
create table classes (id int unsigned auto_increment primary key not null,name varchar(30) not null
);

插入数据

-- 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,'刘德华',59,175.00,1,2,1),
(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;

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1[as 别名1],字段2[as 别名2],…from 表名;
```
select id as 序号, name as 名字, gender as 性别 from students;
```

表名.字段名

select students.id,students.name,students.gender from students;

可以通过 as 给表起别名

select s.id,s.name,s.gender from students as s;
-- 错误:select students.id,students.name,students.gender from students as s

消除重复行
在select后面列前使用distinct可以消除重复的行
select distinct 列1,… from 表名;
```
select distinct gender from students;
```

条件查询

select …from 表名 where …;

比较运算符

等于: =
大于: >
大于等于: >=
小于: <
小于等于: <=
不等于: != 或 <>

-- >
-- 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 "__";
-- 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 (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;
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,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

-- 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

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

分组

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;--显示每组的人是谁
select gender,group_concat(name) 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 对结果进行判断

-- 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

inner join … on(交集)

select … from 表A inner join 表B;

--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(以左边的数据表为基准,没有显示NULL)

-- 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/
创建areas表的语句如下：

    create table areas(aid int primary key,atitle varchar(20),pid int
);

从sql文件中导入数据(数据网上很多自己找一下)

source areas.sql;-- 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

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);
select * from areas where pid = (select aid from areas where atitle="山东省");

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);

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python进阶(十)_mysql数据查询

数据的准备

查询

条件查询

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