mysql数据操作之单表查询

一：介绍

mysql数据操作：DML

在mysql管理软件中，可以通过sql语句中的DML语言来实现数据的操作，包括

1、使用INSERT实现数据的插入

2、UPDATE实现数据的更新

3、使用DELETE实现数据的删除

4、使用SELECT查询数据。

二：插入数据INSERT

1、插入完整数据（顺序插入）
语法一：
insert into 表名（字段1，字段2，字段3，字段n）values (值1，值2，值3)；
语法二：
insert into 表名 values （值1，值2，值3，值n);
2、指定字段插入数据
语法：
insert into 表名（字段1，字段2，字段3.。。) values(值1，值2，值3)；
3、插入多条记录
语法：
insert into 表名 values
    (值1，值2，值n)，
    (值1，值2，值n)，
    (值1，值2，值n)；
4、插入查询结果
    语法：
    insert into 表名（字段1，字段2，字段3，字段n）
                select （字段1，字段2，字段3，字段n）FROM 表2
                where....;

三更新数据UPDATAE

语法：
    UPDATE 表名 SET
        字段1=值1,
        字段2=值2,
        WHERE CONDITION;

示例：
    UPDATE mysql.user SET password=password(‘123’) 
        where user=’root’ and host=’localhost’;

四：删除数据DELETE

语法：
    DELETE FROM 表名 
        WHERE CONITION;

示例：
    DELETE FROM mysql.user 
        WHERE password=’’;

练习：
    更新MySQL root用户密码为mysql123
    删除除从本地登录的root用户以外的所有用户

五：查询数据select

#1:单表查询

一单表查询的语法：

  select distinct 查询字段1，查询字段2，。。。 from 表名
        where 分组之前的过滤条件
        group by 分组依据
        having 分组之后的过滤条件
        order by 排序字段
        limit 显示的条数;

二、关键字的执行优先级（重点）

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，取文件／表取出一条条记录

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、将分组的结果进行having过滤

5、执行select

6、去重

7、将结果按照条件排序；order by

8、限制结果的显示条数

三：简单查询

company.employee
    员工id      id                  int             
    姓名        emp_name            varchar
    性别        sex                 enum
    年龄        age                 int
    入职日期     hire_date           date
    岗位        post                varchar
    职位描述     post_comment        varchar
    薪水        salary              double
    办公室       office              int
    部门编号     depart_id           int

create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
);

创建表

mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | NO | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(3) unsigned | NO | | 28 | | | hire_date | date | NO | | NULL | | | post | varchar(50) | YES | | NULL | | | post_comment | varchar(100) | YES | | NULL | | | salary | double(15,2) | YES | | NULL | | | office | int(11) | YES | | NULL | | | depart_id | int(11) | YES | | NULL | | +--------------+-----------------------+------+-----+---------+----------------+

三个部门：教学，销售，运营 insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values ('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部 ('alex','male',78,'20150302','teacher',1000000.31,401,1), ('wupeiqi','male',81,'20130305','teacher',8300,401,1), ('yuanhao','male',73,'20140701','teacher',3500,401,1), ('liwenzhou','male',28,'20121101','teacher',2100,401,1), ('jingliyang','female',18,'20110211','teacher',9000,401,1), ('jinxin','male',18,'19000301','teacher',30000,401,1), ('成龙','male',48,'20101111','teacher',10000,401,1), ('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门 ('丫丫','female',38,'20101101','sale',2000.35,402,2), ('丁丁','female',18,'20110312','sale',1000.37,402,2), ('星星','female',18,'20160513','sale',3000.29,402,2), ('格格','female',28,'20170127','sale',4000.33,402,2), ('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门 ('程咬金','male',18,'19970312','operation',20000,403,3), ('程咬银','female',18,'20130311','operation',19000,403,3), ('程咬铜','male',18,'20150411','operation',18000,403,3), ('程咬铁','female',18,'20140512','operation',17000,403,3) ; #ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

#简单查询 SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee; SELECT * FROM employee; SELECT name,salary FROM employee; #避免重复DISTINCT SELECT DISTINCT post FROM employee; #通过四则运算查询 SELECT name, salary*12 FROM employee; SELECT name, salary*12 AS Annual_salary FROM employee; SELECT name, salary*12 Annual_salary FROM employee; #定义显示格式 CONCAT() 函数用于连接字符串 SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary FROM employee; CONCAT_WS() 第一个参数为分隔符 SELECT CONCAT_WS(':',name,salary*12) AS Annual_salary FROM employee; 结合CASE语句： SELECT ( CASE WHEN NAME = 'egon' THEN NAME WHEN NAME = 'alex' THEN CONCAT(name,'_BIGSB') ELSE concat(NAME, 'SB') END ) as new_name FROM emp;

select id, name from db39.emp where id >=3 and id <=6 select * from db39.emp where id between 3 and 6; select * from emp where salary =20000 or salary =18000 or salary =17000; select * from emperor where salary in (20000,18000,17000); 要求：查询员工姓名中包含i字母的员工姓名与其薪资 select name, salary from db39.emp where name like '%I%' 要求：查询员工姓名是由四个字符组成的员工姓名与其薪资 select name,salary from db39.emp where name like '____'; select name,salary from db39.emp where char_length(name) = 4; select * from db39.emp where id not between 3 and 6; select * from emp where salary not in (20000,18000,17000); 要求：查询岗位描述为空的员工名与岗位名 select name,post from db39.emp where post_comment is NULL; select name,post from db39.emp where post_comment is not NULL;

#设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据 mysql> set global sql_mode="strict_trans_tables,only_full_group_by"; #每个部门的最高工资 select post,max(salary) from emp group by post; select post,min(salary) from emp group by post; select post,avg(salary) from emp group by post; select post,sum(salary) from emp group by post; select post,count(id) from emp group by post; #group_concat（分组之后用） select post,group_concat(name) from emp group by post; select post,group_concat(name,"_SB") from emp group by post; select post,group_concat(name,": ",salary) from emp group by post; select post,group_concat(salary) from emp group by post; # 补充concat（不分组时用） select name as 姓名,salary as 薪资 from emp; select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪资 from emp; # 补充as语法 mysql> select emp.id,emp.name from emp as t1; # 报错 mysql> select t1.id,t1.name from emp as t1;

having的语法格式与where一模一样，只不过having是在分组之后进行的进一步过滤即where不能用聚合函数，而having是可以用聚合函数，这也是他们俩最大的区别 1、统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门 select post,avg(salary) from emp where age >= 30 group by post having avg(salary) > 10000; #强调：having必须在group by后面使用 select * from emp having avg(salary) > 10000;

select * from emp order by salary asc; #默认升序排 select * from emp order by salary desc; #降序排 select * from emp order by age desc; #降序排 select * from emp order by age desc,salary asc; #先按照age降序排，再按照薪资升序排 # 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行排序 select post,avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary) ;

mysql数据操作之单表查询

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