03 增删改查(重点--记忆)这部分内容过多，未来会重新整理

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03、 增删改查(curd)(重点--记忆)

3.1增加、插入——insert

+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
+-------+------------------+------+-----+---------+----------------+

-- 全列插入
-- insert [into] 表名 values(...)
-- 主键字段 可以用 0  null   default 来占位
-- 向classes表中插入 一个班级
insert into classes values(1,"python11");

+--------+----------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+----------------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | int(10) unsigned | NO | | NULL | |
| high | decimal(5,2) | YES | | NULL | |
| gender | enum(‘男‘,‘女‘,‘保密‘) | YES | | 保密 | |
| cls_id | int(10) unsigned | YES | | NULL | |
+--------+----------------------------+------+-----+---------+----------------+

    -- 向students表插入 一个学生信息
    insert into students values(1,"班主任",18,166.66,2,110);

    -- 部分插入
    -- insert into 表名(列1,...) values(值1,...)
    insert into students(name,age,gender) values("吴彦祖",20,1);

    -- 多行插入
	insert into students values(0,"老谢",30,188.88,1,110),(0,"老苏",28,177.77,1,110);

3.2删除——delete

-- 删除
    -- 物理删除
    -- delete from 表名 where 条件
    delete from students where id = 2;

    -- 逻辑删除
    -- 用一个字段来表示 这条信息是否已经不能再使用了
    -- 给students表添加一个 is_delete 字段 bit 类型
	alter table students add is_delete bit default 0;
	update students set is_delete = 1 where id = 3;
	
-- 数据库备份与恢复(了解)
	-- mysqldump –uroot –p 数据库名 > python.sql;
	-- mysql -uroot –p 新数据库名 < python.sql;

3.3修改——update

-- 修改
-- update 表名 set 列1=值1,列2=值2... where 条件;
    -- 全部修改
    update students set high = 170.00;
	
	-- 按条件修改
	update students set high = 188.88 where id = 2;

	-- 按条件修改多个值
	-- update students set gender ="",name = "xxx" ;

3.4查询基本使用

    -- 查询所有列
    -- select * from 表名;
    select * from students;


    ---定条件查询
    select * from students where id = 2;

    -- 查询指定列
    -- select 列1,列2,... from 表名;
    select name,gender from students;

    -- 可以使用as为列或表指定别名
    -- select 字段[as 别名] , 字段[as 别名] from 数据表;
    select name as "姓名",gender as "性别" from students;

    -- 字段的顺序
    select gender as "性别",name as "姓名" from students;

import xxxxxx as x
-- 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,gender from students;
避免重复字段信息distinct
select distinct name,gender 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",gender as "性别" from students;

-- select 表名.字段 .... from 表名;
select students.name,students.gender from students;


-- 可以通过 as 给表起别名
-- select 别名.字段 .... from 表名 as 别名;
select s.name,s.gender from students as s;

失败的select students.name, students.age from students as s;

-- 消除重复行(查性别)

-- distinct 字段 
select distinct gender from students;

条件查询——where

-- 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;

	-- <
	-- 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
	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;(注意)

	-- != 或者 <>
	select * from students where age != 18;

-- 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;

	失败select * from students where 18<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 = 2;

	-- or
	-- 18以上或者身高高过180(包含)以上
	select * from students where height >= 180 or age > 18;

	-- 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);

	select * from students where not (age > 18 and gender = 2);(注意)

		

-- 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、34的姓名
	select name from students where age in (18,34);

	-- not in 不非连续的范围之内
	-- 年龄不是 18、34岁的信息
	select * from students where age not in (18,34);

	(注意)select name from students where not age 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 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;
	
	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;
	
	-- 判非空is not null
	select * from students where height is not null;
	
	失败select * from students where height not is  null;

排序——order by

-- 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，按照年龄从小到大到排序
select * from students where gender = 1 and (age between 18 and 34) 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 gender = 2 and (age between 18 and 34) 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 gender = 2 and (age between 18 and 34) order by height desc,age asc;

-- 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 gender = 2 and (age between 18 and 34) order by height desc,age asc,id desc;

3.5聚合函数

count maxminsumavg

-- 总数
-- count                                                                                                                                                                                                                                                                                                                           
-- 查询男性有多少人，女性有多少人
select count(*) from students where gender = 1;

-- 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 = 2;

-- 最小值
-- min
select min(height) from students;

(错误的)select min(height),name 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(*)



-- 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=1;

3.6分组(重点)group by

-- group by
-- 按照性别分组,查询所有的性别
select gender from students group by gender;

select name,gender from students group by gender;错误
-- select name,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 count(*),gender 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;

-- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30(重点)
select gender,group_concat(name) from students group by gender having avg(age) > 30;

-- 查询每种性别的平均年龄和名字
select gender,avg(age),group_concat(name) from students group by gender;

-- 查询每种性别中的人数多于2个的性别和姓名（重点）
select gender,group_concat(name) 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 字段,group(avg(字段),int),group_concat(字段) from table group by 字段 having 判断条件语句 order by 字段(int:类型) desc(asc); |
| |

| select (显示字段，求平均，保留int位，从函数中取值) from (数据表) group by (分组字段) having (判断条件例:not (age > 18 and gender = 2)) order by (排序); |

3.7分页limit

-- limit start, count


-- 限制查询出来的数据个数
-- 查询前5个数据
limit 页，数
select * from students limit 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;

错误1 select * from students limit 10,2 order by age asc;

-- 工作错误的写法
错误2 select * from students limit 2*(6-1),2;

-- limit 放在最后面(注意)

3.8连接查询(重点)inner join

-- inner join ... on
-- select ... from 表A inner join 表B;
select * 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 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 students 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 inner join classes on students.cls_id = classes.id;
select * from students left join classes on students.cls_id = classes.id;

-- select * from students right join classes on students.cls_id = classes.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 left join classes on students.cls_id = classes.id where classes.id is null;

(注意)不建议使用 select * from students left join classes on students.cls_id=classes.id having classes.id is null;

-- right join   on
-- 将数据表名字互换位置，用left join完成

3.9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)
-- 1 查出平均身高
select avg(height) from students;
-- 2 查出高于平均身高的信息
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 name from students where cls_id in (select id from classes);
-- 1 查出所有的班级id
select id from classes;
(1,2)
-- 2 查出能够对应上班级号的学生信息
select * from students where cls_id in (select id from classes);

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