数据库查询操作

1.单表查询

--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 name2 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 name from students as s;select s.name from students as s;-- 消除重复行(查性别)-- distinct 字段 
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;-- >=-- <=-- 查询小于或者等于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;-- 逻辑运算符and or not-- 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 not age > 18 and gender = '女';-- 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 '__%';-- 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 name from students where age = 18 or age = 34 or age = 56;select * from students where age in (18,34,56);-- 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 (18,34,56);-- 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-- 包含两端的数据 ,数据需要 从小到大填写select * 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 nullselect * from students where height is not null;-- 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 >= 18 and age <= 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 >= 18 and age <= 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 >= 18 and age <= 34 and gender='女'order by height desc , age asc, id desc;-- 排序有优先级,第一位的优先级最高-- 聚合函数-- 总数-- count-- 查询男性有多少人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-- minselect min(height) from  students where gender = '女';-- 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 avg(height) from students;select sum(height) / count(*) from students;select count(height)  from students;-- sql中null不会进行计算 
select sum(height) / count(height) from students;-- 分组分组的公式 select 分组字段 from 表名 group by 分组的字段;-- 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;-- 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;-- 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),group_concat(name) from students group by genderhaving 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 genderhaving count(*) > 2;-- with rollup 汇总的作用(了解)select gender,count(*) from students group by genderwith rollup having count(*) > 2;-- 分页-- limit start, count-- limit 放在最后面(注意)起始位置 : (页数-1)*每一页个数-- 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 10,2;-- 先排序再分页,分页一定在最后做的

2.多表查询

-- 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 sinner join classes as con 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 classeson 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 classeson 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 classeson 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 classeson students.cls_id = classes.id order by classes.name asc ,students.id asc;-- left join-- 查询每位学生对应的班级信息--左联,就是左边表的数据不管在右边能不能找到都显示 select * from students left join classes on students.cls_id = classes.id;-- 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.id 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 classes.id = students.cls_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 > 172.076923;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 id from classes;select * from students where cls_id in(1,2);select * from students where cls_id in (select id from classes);--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;-- 直接将文件拖入命令行中,获得文件地址,去掉引号即可
source xxx/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 a2on 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 a2on a1.pid = a2.aid where a2.atitle = '广州市';

3.MySql远程连接