单表查询

一、查询语法

SELECT 字段1,字段2... FROM 表名
                  WHERE 条件
                  GROUP BY field
                  HAVING 筛选
                  ORDER BY field
                  LIMIT 限制条数
二、关键字的优先级

重点中的重点：关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit

1.找到表:from

2.拿着where指定的约束条件，去文件/表中取出一条条记录

3.将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组

4.按照select后的字段得到一张新的虚拟表，如果有聚合函数，则将组内数据进行聚合

5.将4的结果过滤：having,如果有聚合函数也是先执行聚合再having过滤

6.查出结果：select

7.去重

8.将结果按条件排序：order by

9.限制结果的显示条数

三、where约束

强调：where是一种约束条件，mysql会拿着where指定的条件去表中取数据，而having则是在取出数据后进行过滤

where字句中可以使用：

1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
pattern可以是%或_，
%表示任意多字符
_表示一个字符
5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

#1:单条件查询
    SELECT name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND
    SELECT name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

    SELECT name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)
    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NULL;

    SELECT name,post_comment FROM employee 
        WHERE post_comment IS NOT NULL;
        
    SELECT name,post_comment FROM employee 
        WHERE post_comment=''; 注意''是空字符串，不是null
    ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了

#5:关键字IN集合查询
    SELECT name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE name LIKE 'eg%';

    通配符’_’
    SELECT * FROM employee 
            WHERE name LIKE 'al__';

表一对一

create table customer(
id int primary key auto_increment,
name varchar(50),
qq int,
phine int
);

insert  into customer(name,qq,phine) values('egon','251575084','1779422095'),
('alex','251575089','1779422558');



create table student(
id int primary key auto_increment,
course varchar(20),
c_id int,
foreign key(c_id) references customer(id)
on delete cascade
on update cascade
);


insert into student(course,c_id) values ('python','5');

表一对多

create table press(
id int primary key auto_increment,
name varchar(20)
);

create table book2(
id int primary key auto_increment,
name varchar(20),
press_id int not null,
foreign key(press_id) references press(id)
on delete cascade
on update cascade
);


insert into press(name) values
('人民文学出版社'),
('人民邮电出版社'),
('科学出版社');

insert into book2(name,press_id) values
('和平',1),
('世界十大名著',2),
('世界上的另一个你',2),
('人性弱点',3),
('爆笑校园',2),
('傲慢与偏见',3);

多对一

create table user(
id int primary key auto_increment,
name varchar(10)
);

create table password(
id int primary key auto_increment,
user_id int ,
password varchar(20),
foreign key(user_id) references password(id)
on update cascade
on delete cascade
);


insert into user(name) values
('egon1'),
('egon2'),
('egon3'),
('egon4'),
('egon5'),
('egon6');


insert into password(user_id,password) values
(3,'alex3714'),
(5,'alex371a');

多对多

create table book(
id int primary key auto_increment,
name varchar(10),
price int
);

create table author(
id int primary key auto_increment,
name varchar(10)
);


create table author2book(
id int primary key auto_increment,
book_id int ,
author_id int, 
foreign key(book_id) references author2book(id)
on delect cascade
on delect cascade,
foreign key(author_id) references author2book(id)
on delect cascade
on delect cascade,
unique(book_id,author_id)
);