sql基础语法入门

实验环境: mysql

DB链接工具：DBeaver(开源工具)

– 基础建表语句：图书编号 书名 作者 性别 价格 供应商:sql语句字符与大小写无关

create table books(

id int primary key auto_increment, – 定义数据表的主键，并声明主键值增长策略

bookName varchar(10), – 字符类型：长度可变

author varchar(10) not null, – 约束不能为空

gendor char(2) check (gendor in (‘M’,’F’)), – 性别约束：男或女

price float, – 浮点型

vendorId int, – 出版社代号

foreign key(vendorId) references vendors(id) – 建立外键约束

)character set utf8 collate utf8_general_ci; – 建表时确认表数据的编码格式

–书籍发布商数据表

create table vendors (

id int primary key auto_increment,

name varchar(100) NOT NULL,

tele varchar(100) NULL,

manager varchar(100) NULL

)character set utf8 collate utf8_general_ci;

– 删除数据表:

drop table books

– 数据表中插入完整字段（初始化sql测试数据）

books表：

insert into books values(1, ‘西游记’, ‘吴承恩’, ‘M’,20.5, 1);

insert into books values(2, ‘三国演义’, ‘罗贯中’,’F’, 49.4,2);

insert into books values(3, ‘水浒传’, ‘施耐庵’,’M’, 23.5, 3);

insert into books values(4, ‘红楼梦’, ‘曹雪芹’, ‘M’, 50.5, 3);

insert into books values(5, ‘史记’, ‘司马迁’, ‘M’, NULL, 2);

insert into books values(7, ‘AAAA’, ‘吴承恩’, ‘M’, 30.21, 3);

vendors表：

insert into vendors values(1,’北京大学出版社’,’020-25325452’,’张三’);

insert into vendors values(2,’清华大学出版社’,’0755-27327552’,’李四’);

insert into vendors values(3,’人民日报出版社’,’010-25322452’,’王五’);

insert into vendors values(4,’南方日报出版社’,’0783-25565452’,’马六’);

– 数据表中传入部分字段：格式：表名后定义需要存放的字段

insert into books(bookName,author,gendor,price,vendorId)

values(‘World War’,’Stepheon’,’F’,25.4,1);

– 删除数据表中的记录

delete from books where id=1;

– 更新数据表记录

update books set gendor=’M’, price=20 where id=1;

– 数据表查询

select *from books;

select *from vendors;

– 查询数据记录的部分字段:字段\表别名：as关键字可缺省

select bookName 书名,author as 作者,price 价格 from books allBooks where allBooks.id=1;

– 条件语句的使用where:and/or

select *from books where price>23; – 算术表达式：查询价格大于23的数据

select *from books where (2>1) and (3>2); – 逻辑表达式：and只有两个条件都满足才为真

select *from books where (1>2) or (2>1) ; – or：两个条件只要有一个满足即为真

– and和or的优先级：and的优先级大于or

select *from books where gendor=’M’ and id=1 or price>50;

– 等价于：(gendor=’M’ and id=1) or price>50:相当于合并满足两个条件的结果

– and和or的优先级问题在进行sql拼接时容易产生sql注入漏洞

select *from books where gendor=’M’ and 1 or 1=1; – 等价于：(gendor=’M’ and 1) or 1=1:条件永远为真

– sql统计函数

select min(price),price from books; – 查询价格的最小值

select max(price),price from books; – 查询价格的最大值

select avg(price) from books; – 查询价格的平均值

select sum(price) from books; – 查询价格的总价

select ucase(bookName) from books; – 将字段值转换为大写字母

select lcase(bookName) from books; – 字段值转换为小写字母

select length(bookName) from books where id=6; – 获取字段的大小

select round(price) from books where id=2; – 四舍五入

– 格式化函数

>

select date_format(now(),’%Y-%m-%d’),allBooks.* from books as allBooks;–日期格式化

select formate(price,1),allBooks.* from books as allBooks; – 数字截取：最后一位四舍五入

select truncate(price,1),allBooks.* from books as allBooks; – 数字截取：其他位直接舍弃

– order by排序:默认升序排列

select *from books order by price;

select *from books order by price asc; – 升序：默认

select *from books order by price desc; – 降序：默认

– group by分组:通过不同的值划分不同的组,取出值不同的记录：默认按分组字段的升序排列

select *from books group by gendor order by id;

– select *from books ：等价于

select *from books group by id – 因为主键必定不相同，安主键分组得到的数据就是数据表所有的记录

– group by –having联合使用:筛选出书籍单价总额大于60的男性作者信息

select author 作者,sum(price) 总价,gendor 性别 from books where gendor=’M’ group by author having sum(price)>60

– count(*)：满足条件的记录数

– count(字段名)：满足条件的记录的特定字段非NULL值行的个数： count(*)>=count(字段数)

select count(*) from books where gendor=’M’; – 查询满足条件的记录的条数(行数)：5

select count(price) from books where gendor=’M’; – 查询满足添加的记录中字段值的个数:4

– 限制获取的记录数

select *from books where gendor=’M’ limit 2; – 表示记录数

select *from books limit 1,2; – 表示从满足记录的第1记录之后的连续两条记录(不包含第一条)