一、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 databases(); #报错,因为多加了s
select database();
--创建一个数据表
create table students(
id int unsigned not null auto_increment primary key,
name varchar(20) default '',
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
);
create table classes(
id int unsigned auto_increment primary key not null,
name (30)
);
--查看一下是否创建成功
show tables;
show create table students;
--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,"金兴",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中插入数据
insert into classes values (0,"python_01级"),(0,"python_02级"),(0,"python_03级");
二、查询
2.1普通 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为字段指定别名
--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;
--通过as 给表名起字段
--select 别名.字段 ... from 表名 as 别名;
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
可以用于显示唯一值
--distinct 字段
select distinct gender from students;
2.2条件查询
比较运算符
--select...from 表名 where .....
-->=大于等于; <=小于等于; !=不等于
select * from students where age>=18;
select id,name,gender from students where age>=18;
逻辑运算符
-and
--18到28之间的所有学生信息
select * from students where age>18 and age<28;
select * from students where age>18 and gender="女";
-or
--18以上或身高过180(包含)以上的
select * from students where age>18 or height>=180;
-not
--不在 18岁以上的女性 这个范围信息##注意下面的区别
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
-%替换一个或多个
-_替换一个
--查询姓名中 以“小”开始的名字
select name from students where name like "小%";
--查询姓名中 有“小”的所有的名字
select name from students where name like "%小%";
--查询是两个字的名字
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 周开始的名字
select name from students where name rlike "^周.*";#正则表达式还记得多少
--查询以 周开始伦结尾的名字
select name from students where name rlike "^周.*伦";
范围查询
-in(1,3,8)表示在一个非连接的范围内
--查询年龄为12,18、34的姓名
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12,18,34岁时间的信息
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之间的信息
select name,age 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;
-判非空is not null
select * from students where height is not null;
2.3 排序
-order by 字段
-asc从小到大排列,升序,默认升序
-desc从大到小排序,降序
--查询年龄18到34之间的男性,按年龄从小到大排序
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 asc;
--查询年龄18到34之间的女性,按身高从高到矮排序
select * from students where (age between 18 and 34) and gender=2 order by high desc;
--order by 多个字段
--查询年龄在18到34之间的女性,按身高从高到矮排序,如果身高相同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 high desc,age asc;
--查询年龄在18到34之间的女性,按身高从高到矮排序,如果身高相同情况下按照年龄从小到大排序,如果年龄也相同按id从大到小排序
select * from students where (age between 18 and 34) and gender=2 order by high desc,age asc,id desc;
--按照年龄从小到大、身高从高到矮排序
select * from students order by age asc,high desc;
2.4聚合函数
2.4.1 聚合
-总数 count
--查询男性有多少人,女性有多少人
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 最小值 min
--查询最大年龄
select max(age) from students;
--查询女性的最高 身高
select max(high) from students where gender=2;
-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)--> 保留一位小数
--计算所有人的平均年龄,保留2位小数
select round(sum(age)/count(*),2) from students;
--计算男性的平均身高, 保留两位小数
select round(svg(high),2) from students where gender=1;
2.4.2 分组 (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(....)
--查询同种性别中的具体姓名
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!!!!是对分组进行条件判断!!!where是对原表进行条件判断
--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) from students
group by gender having avg(age)>30;
--查询每种性别的人数多于两个的信息
select gender,group_concat(name) from students group by gender having conut(*)>2;
group_concat()用法
2.5分页
-limit start,count
--限制查询出来的数据个数
select * from students where gender=1 limit 2;
--查询前五个数据
select * from students limit 0,5;
--查询 id6-10(包含)的书序
select * from students limit 5,5;
so!--->>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
--失败select * from students limit 2*(6-1),2;--->因为limit 2*(6-1)不可
--失败select * from students limit 10,2 order by age asc;--->因为limit要放最后
select * from students order by age asc limit 10,2;
2.6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##上面c.id列显得多余,此命令去除
select students.*,classes.name from students
inner join classes on students.cls_id=classes.id;
--给数据表起名字##减少输入,可读性也变强了
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.xxx s.xxx from student as s
--inner join classes 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;
--当是同一个班级的时候,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;
--查询没有对应班级信息的学生
--select ... from xxx left join xxx on ...where ...
--select ... from xxx left join xxx 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;
2.7 自关联
首先创建一个表t_d_areainfo,导入全国省市区行政编码,下面进行自关联查询
创建表:
CREATE TABLE `t_d_areainfo` (
`id` int(11) NOT NULL,
`name` varchar(48) NOT NULL DEFAULT '' COMMENT '名称',
`arealevel` tinyint(2) NOT NULL DEFAULT '0'
COMMENT '层级标识: 1 省份, 2 市, 3 区县',
`p_id` int(11) DEFAULT NULL COMMENT '父节点',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='区县行政编码字典表';
--查询所有省份
select * from t_d_areainfo where p_id is null ;
-查询安徽省--下属市--下属区
select id from t_d_areainfo where name="安徽省";
--得到安徽省的id 为340000,继续查询得到下属市
select * from t_d_areainfo where p_id=340000;
--得到芜湖市的id为 340200,继续查询得到下属区
select * from t_d_areainfo where p_id=340000;
直接查询安徽省的市
select province.name,city.name from t_d_areainfo as province
inner join t_d_areainfo as city on city.p_id=province.id
having province.name="安徽省";
也可以
select province.name,city.name from t_d_areainfo as province
inner join t_d_areainfo as city on city.p_id=province.id
having province.name="芜湖市";
2.8子查询
在一个select语句中,嵌入另外一个select语句,那么被嵌入的select语句称为子查询语句
主查询:主要查询的对象,第一条select语句
主查询和子查询的关系
- 子查询是嵌入到主查询中
- 子查询是辅助主查询的,要么充当条件,要么充当数据源
- 子查询可以是独立存在的语句,也可以是一条完整的select语句
select * from students where high =(select max(high) from students);