【数据库】——DML数据操作语言的定义与使用

上一篇文章讲了关于DDL数据定义语言的相关操作，这篇文章，就来好好看看有关于数据库数据操作语言的定义与使用吧~

一、添加数据——insert

首先，我们创建一个学生表。

create table stu
(
	id varchar(15) primary key comment "学生编号",
	name varchar(20) not null comment "学生姓名",
	sex enum('man','woman') default 'man' comment "学生性别",
	age int default 22 comment "学生年龄"
);

结果如下图所示：

1、插入全部属性的数据，不带属性
insert into stu values(“001”,‘zhangsan’,‘man’,20);

2、插入全部属性的数据，要带属性
insert into stu(id,name,sex,age) values(“001”,‘zhangsan’,‘man’,20);

3、插入个别属性的数据
错误方式！
insert into stu values(“002”,“lisi”,“woman”);属性没有一一对应
改进：insert into stu(id,name,sex) values(“002”,“lisi”,“woman”);

4、小批量的插入数据

nsert into stu values("002","lisi","woman",19),
	                             ("003","wangwu","man",22),
                                 ("004","zhaoliu","man",21),
                                   ("005","kaixin","woman",18),
                                   ("006","gaoxin","man",23);
                                  

二、删除数据——delete

1、删除所有数据
delete from stu;

2、过滤条件
delete from stu where id = “006”;

三、修改数据——updata

1、所有满足
update stu set sex = “man”;

2、过滤条件
update stu set age = 19 where id = “002”;

四、查询数据——select

1、普通查询
1.1有通配符的查询：不建议使用
select * from stu;

2、具体查询某个字段
select id, name, sex, age from stu;效果同上图

3、过滤条件
select * from stu where id = “001”;

2、去重查询 distinct
为了解决数据过大冗余问题
select distinct age from table_name;

3、排序 order by
升序 asc 降序 desc ，默认是升序
select distinct age from stu order by age;

以上的查询都是单表查询，接着我们要引出等值查询、连接查询等概念，所以我们在此再创建一个学生成绩表result.
result相关信息如下：

insert into result values("001","p01",78),
("001","p02",56),
("002","p01",34),
("002","p02",89),
("003","p01",78),
("003","p02",57);

4.分组查询 group by
查询每个学生的课程总分
分析：


select id,Sum(score) from result group by id;

5、连接查询
查询年龄小于20岁学生的不及格成绩
如果直接连接查询的话
select stu.id,score
from stu,result
where stu.id = result.id and age < 20 and score < 60;
分析：这样他会按照笛卡尔乘积的方式来查找数据，如下图，效率非常低

5.1外连接查询
5.1.1左外连接查询：左表过滤的结果必须全部存在，

select a.id,score 
					from
					(select id,age from stu where age < 20) a 
					left join 
					(select id, score from result where score < 60) b
					on a.id = b.id
					where b.score is not null;

5.1.2右外连接查询：左表过滤的结果必须全部存在

select a.id,score 
					from 
					(select id,age from stu where age < 20) a
					right join
					(select id,score from result where score < 60) b
					on a.id = b.id

5.1.3全外连接查询

select a.id,score 
					from 
					(select id,age from stu where age < 20) a
					full join
					(select id,score from result where score < 60) b
					on a.id = b.id
					where a.id is not null;

5.2 内连接查询 只筛选匹配项

select a.id,score 
					from 
					(select id,age from stu where age < 20) a
					inner join
					(select id,score from result where score < 60) b
					on a.id = b.id;

6、聚合查询：union 他有自动去除重复项的功能
查询全校师生的信息

select * from stu
union
select * from teach;