详解SQL中的DQL查询语言

select  from student;
– 查询英语在70到75之间的学生的信息
-- select  from student where english BETWEEN 70 AND 75;

– 查询语文是80或者82或者90分的学生信息
-- select  from student where chinese IN(80,82,90);

– 查询所有首字母为l的学生的成绩
-- select  from student where name like "l%";

– 查询数学大于80且语文大于80 的同学
-- select  from student where math>80 and chinese>90;

– 对数学成绩排序后输出 （默认升序 ASC）
-- select  from student order by math;

– 对数学成绩排序后输出（降序 DESC）
-- SELECT  FROM student order by math DESC;

– 指定多个字段进行排序，先按第一个字段进行排序，如果相同则按第二个字段进行排序
  -- SELECT  FROM student ORDER BY math DESC,chinese DESC;

– WHERE后可以加 ORDER BY
-- SELECT  from student where name like "%l" ORDER BY math DESC;

– 显示student 表格中的前3行
SELECT  from student LIMIT 2;

– 显示student 表格中的第3~5行
SELECT  from student LIMIT 2,3; -- 2表示偏移量，3表示显示的行数

②select |{column1|expression、column2|expression,…}from table;
select column as 别名 from table;

create table customer(
 id int PRIMARY KEY auto_increment,
 name varchar (20) not null,
 adress varchar (20) not null
);
create table orders(
 order_num varchar(20) PRIMARY KEY,
 price FLOAT not NULL,
 customer_id int, -- 进行和customer 关联的字段 外键
 constraint cus_ord_fk foreign key (customer_id) REFERENCES customer(id)
);
insert into customer(name,adress) values("zs","北京");
insert into customer(name,adress) values("ls","上海");
SELECT  from customer;
INSERT INTO orders values("010",30.5,1);
INSERT INTO orders values("011",60.5,2);
INSERT INTO orders values("012",120.5,1);
SELECT  from orders;

primary key 主键 （auto_increment 设置自动增长）
-- UNIQUE 表约束 唯一性标识
-- PRIMARY KEY 主键 
CREATE TABLE t4 (
 id INT PRIMARY KEY auto_increment,
 NAME VARCHAR (20) NOT NULL,
 gender CHAR (5) NOT NULL,
 idCard VARCHAR (20) UNIQUE -- UNIQUE 唯一性标识
);
desc t4;
insert into t4 (name,gender,idCard) VALUE("zs","man","110");
insert into t4 (name,gender,idCard) VALUE("ls","woman","112");