MySql学习笔记——数据库查询语言（DQL）

DQL

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1 查询数据

【在navicat中完成】
  右击新建查询
  

• 查询表中所有数据

select * from students;

  

• 查询指定列

select sname,age from students;

  

select * from students
where sex='nan' and age =16;

  
在navicat中可以保存查询语句方便下次使用
  

select * from students
where sno=1 or sname='wc';

  
  navicat中注释 ctrl+/

• IN查询同一属性的多值,固定范围值

select * from students where sno in(1,2,3);
等同于 select * from students where sno=1 or sno=2 or sno=3;

  
  

• 查询sname值为null的记录

select * from students where sname is null;

  

• 查询sname值不为null的记录

select * from students where sname is not null;

  

• 查询非值的数据

select * from students where sex!='nan';

  

• 查询两者之间的数据

select * from students where age>=5 and age<=12;
等同于select * from students where age between 5 and 12;

  
  

2 模糊查询

• 查询姓名由5个字母构成的学生记录

select * from students where sname like '_____';
-- 五个“_”

  

• 查询姓名为5个字母且最后一个字母是x的学生记录

select * from students where sname like '____x';

  

• 以m开头的学生

select * from students where sname like 'm%';

  

• 第二个字母是s的

select * from students where sname like '_s%';

  

• 包含s的

select * from students where sname like '%s%';

  

3 字段控制查询

• 去除重复查询操作distinct

select distinct name from stu;

  

• 对字段结果进行运算来生成新的字段

select *, age+score from students;

  

• 若值是null则变为0来进行运算

select *, ifnull(age,0)+ifnull(score,0) from students;

• 改变新字段列名

select *, ifnull(age,0)+ifnull(score,0) as total from students;

  

select sname as resname from students;

  
  

• navicat中创建表

create table emp(
 id int(11) not null,
 name varchar(50) default null,
 gender varchar(1) default null,
 hire_date date default null,
 salary decimal(10,0) default null,
 performance double(255,0) default null,
 manage double(255,0) default null,
 department varchar(255) default null
 );
 -- decimal 定义时划定整数和小数的位数，decimal(10,0)指10位整数，0位小数

  
在“表”处右击刷新，看到我们新建的表
  

• navicat中查看建表语句

CREATE TABLE `emp` (
  `id` int(11) NOT NULL,
  `name` varchar(50) DEFAULT NULL,
  `gender` varchar(1) DEFAULT NULL,
  `hire_date` date DEFAULT NULL,
  `salary` decimal(10,0) DEFAULT NULL,
  `performance` double(255,0) DEFAULT NULL,
  `manage` double(255,0) DEFAULT NULL,
  `department` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

4 排序

• 升序（排序默认升序）
  以薪水升序排列

select * from employee order by salary;
等同于select * from employee order by salary ASC;

• 降序
  以薪水降序排列

select * from employee order by salary DESC;

• 多重排序
  当出现相同值时，采用多重排序

select * from employee order by salary DESC,id DESC;

5 聚合函数

• 计数
  数据条数

select conut(*) from employee;

  

• 条件计数
  薪水大于2500的数据条数

select count(*) from employee where salary>2500;

  
  薪水加绩效大于5000的数据条数

select count(*) from employee 
where ifnull(salary,0) + ifnull(performance,0)>5000;

  

• 多条计数

select count(performance),count(manage) from employee;

  

• 求和

select sum(salary) from employee;

  

• 多条求和

select sum(salary),sum(performance) from emplyoee;

  
  

• 求多条和

select sum(salary+ifnull(performance,0)) from emplyoee;

  

• 求平均

select avg(salary) from employee;

  

• 求最大

select max(salary) from employee;

  

• 求最小

select min(salary) from employee;