文章目录
0.数据的准备
1.MySQL查询
2.条件查询
2.1比较运算符
2.2逻辑运算符
2.3模糊查询
2.4 范围查询
3 排序
4 聚合函数
5 分组
6 分页
7 连接查询
8 自关联
9 子查询
0.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那个？
select database();
创建一个数据表
create table students(
id int unsigned not null auto_increment primary key,
name varchar(30),
age tinyint unsigned default 0,
high 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
);
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
);
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”),
(0, “python_04期”);
1.MySQL查询
查询所有字段
select * from 表名;
select * from students;
select id,name from classes;
查询指定字段
select 列1,列2,… from 表名;
select age,name from students;
使用as给字段起别名
select name as 姓名, age as 年龄 from students;
select age as 年龄, name as 姓名 from students;
select 表名.字段 … from 表名;
select students.name, students.age from students;
select s.name,s.age from students as s;
select students.name, students.age from students as s;(失败)
消除重复行
distinct 字段
select distinct gender from students;
2.条件查询
2.1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 stundents where age<18;
=
<=
查询小于或者等于18岁的信息
select * from stundents where age=18;
=
查询年龄为18岁的所有学生的名字
!= 或者 <>
2.2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;
2.3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$”;
2.4 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;
3 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;
4 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 name, round(avg(height), 2) from students where gender=1;
5 分组
group by
– 按照性别分组,查询所有的性别
失败select name from students group by gender;
失败select * from students group by gender;
select gender 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;
select gender,count(* ),group_concat(name) as 人数 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;
6 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;
7 连接查询
内连接 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;
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 as 学生, c.name as 班级 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
– 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;
select c.name,s.* from classes as c left join students as s on s.cls_id = c.id order by c.name,s.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
8 自关联
– 省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/
create table booktest_areainfo(
aid int primary key,
atitle varchar(20),
pid int
);
– 查询所有省份
select * from booktest_areainfo where pid is null;
– 查询出山东省有哪些市
select * from booktest_areainfo as province inner join booktest_areainfo as city on city.pid=province.aid having province.atitle=“山东省”;
select province.atitle, city.atitle from booktest_areainfo as province inner join booktest_areainfo 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 booktest_areainfo 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”)
9 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);
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);
