--数据库操作前的准备 -- 创建数据库 -- create database python_test_1 charset=utf8;
-- 使用数据库 -- use python_test_1;
-- 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 -- );
-- 查询练习 -- 查询所有字段 -- select * from 表名; select * from students;
-- 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 from students;
-- select 表名.字段 .... from 表名; select students.name from students; -- 可以通过 as 给表起别名 -- select 别名.字段 .... from 表名 as 别名; select * from students as s;
select s.name from students as s;
-- 消除重复行(查性别) -- distinct 字段 不要记有个印象 select distinct gender from students;
> < >= <= != <> -- 条件查询 -- 比较运算符 -- select .... from 表名 where ..... -- > -- 查询年纪大于18岁的信息 select * from students where age > 18;
-- < -- 查询年纪小于18岁的信息 select * from students where age < 18;
-- >= -- <= -- 查询小于等于18岁的信息 select * from students where age <= 18;
-- = -- 查询年龄为18岁的所有学生的名字 select * from students where age = 18;
-- != 或者 <> -- 查询年龄不为18岁的所有学生的名字 select * from students where age != 18; -- select * from students where age <> 18; and or not -- 逻辑运算符 -- and -- 18和28之间的所以学生信息 select * from students where age > 18 and age < 28;
-- 18岁以上的女性 select * from students where age > 18 and gender = '女';
-- or -- 18以上或者身高高过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= "女";
select * from students where not (age > 18 and gender = "女"); like % _ -- 模糊查询(where name like 要查询的数据) -- like -- % 替换任意个 -- _ 替换1个 -- 查询姓名中 以 "小" 开始的名字 select * from students where name like '小%';
-- 查询姓名中 有 "小" 所有的名字 select * from students where name like '%小%';
-- 查询有2个字的名字 select * from students where name like '__';
-- 查询有3个字的名字 select * from students where name like '___';
-- 查询至少有2个字的名字 select * from students where name like '__%';
select * from students where name not like "__";
-- 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或34的姓名 select * from students where age = 18 or age = 34 ; select * from students where age in (18,34);
-- not in 不非连续的范围之内 -- 年龄不是 18或34岁的信息 select * from students where age not in(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 * from students where age > 18 and age < 34; -- between xxx and xxx select * from students where age between 18 and 34; --between...and...这是包含两端的数据
-- not between ... and ...表示不在一个连续的范围内 -- 查询 年龄不在18到34之间的的信息
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;
-- 判非空is not null select * from students where height is not null; order by 字段 asc,desc
-- 排序 -- order by 字段 -- asc -- asc从小到大排列，即升序 -- desc -- desc从大到小排序，即降序 -- 查询年龄在18到34岁之间的男性，按照年龄从小到大到排序 select * from students where (age between 18 and 34) and gender='男' 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 ='女' 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 = '女' order by height desc,age asc;
-- 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 ='女' order by height desc,age asc,id desc; -- 排序有优先级,第一个主排序,后面是次排序,在保证主排序不变的情况下,能排就排,不排就算了
-- 聚合函数 -- 总数 -- count -- 查询男性有多少人 count(字段) 要注意如果值有null那么不会进行计算 select count(*) from students where gender='男'; -- 最大值 -- max -- 查询最大的年龄 select max(age) from students; -- 查询女性的最高 身高 select max(height) from students where gender ='女';
-- 最小值 -- min select min(age) from students ;
-- 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 (avg(age),2) 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='男'; select avg(height) from students where gender = '男';
-- 分组
-- group by -- 按照性别分组,查询所有的性别 -- select 分组字段 from 表名 group by 分组字段; select gender from students group by gender;
select 分组字段 from 表名 group by 分组字段;
-- 计算每种性别中的人数 select gender,count(*) from students group by gender;
-- group_concat(...) -- 查询同种性别中的姓名 select gender,group_concat(name) from students group by gender;
-- 查询每组性别的平均年龄 select gender,avg(age) from students group by gender;
-- select * from students where -- group by xxx having having用在分组条件
-- having(注意having和group by 连用 having后通常也要跟 聚合函数) -- 查询平均年龄超过30岁的性别，以及姓名 select gender ,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,count(*) from students group by gender having count(*) > 2;
-- with rollup 汇总的作用(了解) --select gender,count(*) from students group by gender with rollup; select gender,count(*) from students group by gender with rollup having count(*) >2;
--按性别分组,平均身高大160的女性组的名字 select gender,avg(height),group_concat(name) from students group by gender having avg(height) > 160 and gender='女';
-- limit 起始位置,个数, 这个一定要放在最后 -- 分页 -- limit start, count -- limit 放在最后面(注意)
起始位置 = (页数-1)*每页的个数 -- 限制查询出来的数据个数 -- 查询前5个数据 select * from students limit 0,5;
-- 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;
-- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序 select * from students order by age asc limit 6,2; -- 如果重新排序了,那么会显示第一页
-- 连接查询 -- inner join ... on -- select ... from 表A inner join 表B; -- 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.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 students.*，只显示班级名称 classes.name. select students.* ,classes.name from students inner join classes on students.cls_id = classes.id;
-- 在以上的查询中，将班级名显示在第1列 select classes.name,students.* from students inner join classes on students.cls_id = classes.id;
-- 查询 有能够对应班级的学生以及班级信息, 按照班级名进行排序 select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc;
-- 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 classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc,students.id asc;
#如果是group by 条件使用having #如果是inner join条件使用on #其他都用where
-- left join -- 查询每位学生对应的班级信息 select * from students left join classes on students.cls_id = classes.id; 左边的表不管在右边的表中是否找到数据,都显示
-- 查询没有对应班级信息的学生 select * from students left join classes on students.cls_id = classes.id where classes.name is null;
-- right join on -- 将数据表名字互换位置，用left join完成
select * from students right join classes on students.cls_id = classes.id;
select * from classes right join students on students.cls_id = classes.id;
-- 子查询 -- 标量子查询: 子查询返回的结果是一个数据(一行一列) -- 列子查询: 返回的结果是一列(一列多行) -- 行子查询: 返回的结果是一行(一行多列) -- 查询出高于平均身高的信息(height) select avg(height) from students;
select * from students where height > 172;
select * from students where height > (select avg(height) from students);
-- 查询学生的班级号能够对应的 学生名字 select * from students where cls_id in (1,2);
select id from classes;
select * from students where cls_id in (select id from classes); 省市区三级联动 --数据操作前的准备 --创建数据库表 create table areas( aid int primary key, atitle varchar(20), pid int ); --从sql文件中导入数据 -- source 具体地址/areas.sql; source areas.sql;
--查询一共有多少个省 select * from areas where pid is null;
--例1：查询省的名称为“山西省”的所有城市 select aid from areas where atitle = '山西省'; select * from areas where pid = (select aid from areas where atitle = '山西省');
select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle='山西省';
--例2：查询市的名称为“广州市”的所有区县 select * from areas where pid = (select aid from areas where atitle = '广州市'); select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle='广州市';
