数据库中总结2

单表查询
单表查询的语法：

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

关键字的执行优先级：

from > where > group by > having > select > distinct > order by > limit

简单查询

 

create table employee(
　　id int not null primary key auto_increment,
　　name char(16) not null,
　　sex enum('male','female') not null default 'male',
　　age int(3) unsigned not null default 28,
　　hire_date date not null,
　　post char(50),
　　post_comment char(100),
　　salary double(15,2),
　　office int,
　　depart_id int	
);

insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','教导主任',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)
;

#简单查询
select id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 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;


查出所有员工的名字，薪资,格式为<名字:egon>,<薪资:3000>
select concat('<名字:',name,'> ','<薪资:',salary,'>') from employee;

查出所有的岗位（去掉重复） 
select distinct depart_id from employee;

查出所有员工名字，以及他们的年薪,年薪的字段名为annual_year
select name,salary*12 as annual_salary from employee;

where 约束
where字句中可以使用：

比较运算符：><>= <= <> !=
between 80 and 100 值在10到20之间
in(80,90,100) 值是10或20或30
like 'egon%'
pattern可以是%或_，%表示任意多字符，_表示一个字符
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

#执行
update employee set post_comment='' where id=2;
select name,post_comment from employee where post_comment='';#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__';

 

 

分组查询：group by
分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等

小窍门：‘每’这个字后面的字段，就是我们分组的依据
1.单独使用GROUP BY关键字分组
SELECT post FROM employee GROUP BY post;
2.GROUP BY关键字和GROUP_CONCAT()函数一起使用
SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;
3.GROUP BY与聚合函数一起使用
select post,count(id) as count from employee group by post;#按照岗位分组，并查看每个组有多少人	

设置ONLY_FULL_GROUP_BY
set global sql_mode='ONLY_FULL_GROUP_BY';
quit #设置成功后，一定要退出，然后重新登录方可生效

聚合函数
聚合函数聚合的是组的内容，若是没有分组，则默认一组

SELECT COUNT(*) FROM employee;
SELECT COUNT(*) FROM employee WHERE depart_id=1;
SELECT MAX(salary) FROM employee;
SELECT MIN(salary) FROM employee;
SELECT AVG(salary) FROM employee;
SELECT SUM(salary) FROM employee;
SELECT SUM(salary) FROM employee WHERE depart_id=3;

having过滤

执行优先级从高到低：where > group by > having 
#1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。
#2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数
select @@global.sql_mode;
select * from employee where salary > 100000;
select * from employee having salary > 100000;#错误

select post,group_concat(name) from employee group by post having salary > 10000;#错误，分组后无法直接取到salary字段
select post,group_concat(name) from employee group by post having avg(salary) > 10000;

 

查询排序：order by
按单列排序

SELECT * FROM employee ORDER BY salary;
SELECT * FROM employee ORDER BY salary ASC;
SELECT * FROM employee ORDER BY salary DESC;　

按多列排序:先按照age排序，如果年纪相同，则按照薪资排序

SELECT * from employee
ORDER BY age,
salary DESC;

1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序
select * from employee order by age asc,hire_date desc;
2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列	
select post,avg(salary) from employee group by post 
having avg(salary)>10000 order by avg(salary) desc;

 

限制查询记录数

SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默认初始位置为0

SELECT * FROM employee ORDER BY salary DESC
LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条

SELECT * FROM employee ORDER BY salary DESC
LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条

 

使用正则表达式查询

select * from employee where name REGEXP '^ale';

select * from employee where name REGEXP 'on$';

select * from employee where name REGEXP 'm{2}';

查看所有员工中名字是jin开头，n或者g结果的员工信息
select * from employee where name regexp '^jin.*[gn]$';

 

 

多表连接查询
准备表

#建表
create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

查询表
desc department;
desc employee;
select * from department;
select * from employee;

 

多表连接查询
外链接语法

select 字段列表
　　from 表1 inner|left|right join 表2
　　　　on 表1.字段 = 表2.字段;

交叉连接：不适用任何匹配条件。生成笛卡尔积

select * from employee,department;

 

内连接：只连接匹配的行

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department 
where employee.dep_id=department.id;

select employee.id,employee.name,employee.age,employee.sex,department.name from employee 
inner join department on employee.dep_id=department.id;

 

外链接之左连接：优先显示左表全部记录

#以左表为准，即找出所有员工信息，当然包括没有部门的员工
select employee.id,employee.name,department.name as depart_name from employee 
left join department on employee.dep_id=department.id;

 

外链接之右连接：优先显示右表全部记录

select employee.id,employee.name,department.name as depart_name from employee 
right join department on employee.dep_id=department.id;　　

 

全外连接：显示左右两个表全部记录

#注意：mysql不支持全外连接 full JOIN
select * from employee 
left join department on employee.dep_id = department.id
union
select * from employee 
right join department on employee.dep_id = department.id
;
#注意 union与union all的区别：union会去掉相同的纪录

 

子查询

#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等

带IN关键字的子查询

#查询平均年龄在25岁以上的部门名
select id,name from department
where id in 
(select dep_id from employee group by dep_id having avg(age) > 25);

#查看技术部员工姓名
select name from employee
where dep_id in 
(select id from department where name='技术');

#查看不足1人的部门名
select name from department
where id in 
(select dep_id from employee group by dep_id having count(id) <=1);

 

带比较运算符的子查询

#查询大于所有人平均年龄的员工名与年龄
select name,age from employee where age > (select avg(age) from employee);

#查询大于部门内平均年龄的员工名、年龄
select t1.name,t1.age from emp t1
inner join 
(select dep_id,avg(age) avg_age from emp group by dep_id) 
t2 on t1.dep_id = t2.dep_id
where t1.age > t2.avg_age;

带EXISTS关键字的子查询
EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。
而是返回一个真假值。
True或False当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询

select * from employee
where exists
(select id from department where id=203);#结果为True 若把id=205，结果为false

　　

视图

是一个虚拟表，其内容由查询定义。同真实的表一样，视图包含一系列带有名称的列和行数据

视图有如下特点; 
　　1. 视图的列可以来自不同的表，是表的抽象和逻辑意义上建立的新关系。 
　　2. 视图是由基本表（实表）产生的表（虚表）。 
　　3. 视图的建立和删除不影响基本表。 
　　4. 对视图内容的更新（添加、删除和修改）直接影响基本表。 
　　5. 当视图来自多个基本表时，不允许添加和删除数据。　　

创建视图
create view 视图名称 as sql 查询语句 
使用视图
select * from 视图名称;
修改视图
alter view 视图名称 AS SQL语句
删除视图
drop view 视图名称;

create view emp_view as select * from employee;
select * from emp_view;
update emp_view set name='blueky' where id=5; 
select * from employee;#epmloyee表中的字段也变成blueky	
alter view emp_view as select * from employee where age>28;
drop view emp_view;

 

 

触发器：监视某种情况，并触发某种操作。

触发器创建语法四要素：1.监视地点(table)
　　　　　　　　　　　2.监视事件(insert/update/delete)
　　　　　　　　　　  3.触发时间(after/before)
　　　　　　　　　　　4.触发事件(insert/update/delete)

创建触发器语法

create trigger triggerName after/before insert/update/delete
on 表名 for each row #这句话是固定的
begin
#需要执行的sql语句
end
#注意1:after/before: 只能选一个 ,after 表示 后置触发, before 表示前置触发
#注意2:insert/update/delete:只能选一个　

#准备表
CREATE TABLE cmd (
id INT PRIMARY KEY auto_increment,
USER CHAR (32),
priv CHAR (10),
cmd CHAR (64),
sub_time datetime, #提交时间
success enum ('yes', 'no') #0代表执行失败
);

CREATE TABLE errlog (
id INT PRIMARY KEY auto_increment,
err_cmd CHAR (64),
err_time datetime
);

#创建触发器
delimiter //
CREATE TRIGGER tri_after_insert_cmd AFTER INSERT ON cmd FOR EACH ROW
BEGIN
IF NEW.success = 'no' THEN #等值判断只有一个等号
INSERT INTO errlog(err_cmd, err_time) VALUES(NEW.cmd, NEW.sub_time) ; #必须加分号
END IF ; #必须加分号
END//
delimiter ;

#往表cmd中插入记录，触发触发器，根据IF的条件决定是否插入错误日志
INSERT INTO cmd (
USER,
priv,
cmd,
sub_time,
success
)
VALUES
('egon','0755','ls -l /etc',NOW(),'yes'),
('egon','0755','cat /etc/passwd',NOW(),'no'),
('egon','0755','useradd xxx',NOW(),'no'),
('egon','0755','ps aux',NOW(),'yes');

#删除触发器
drop trigger tri_after_insert_cmd;

事务
事务用于将某些操作的多个SQL作为原子性操作，一旦有某一个出现错误，即可回滚到原来的状态，从而保证数据库数据完整性。

事物的特性：原子性、一致性、隔离性、持久性。

在 MySQL 中只有使用了 Innodb 数据库引擎的数据库或表才支持事务。
事务处理可以用来维护数据库的完整性，保证成批的 SQL 语句要么全部执行，要么全部不执行。
事务用来管理 insert,update,delete 语句
SET AUTOCOMMIT=0 ;禁止自动提交 和 SET AUTOCOMMIT=1 开启自动提交.

数据锁
当并发事务同时访问一个资源时，有可能导致数据不一致，就需要一种机制来将数据访问顺序化，以保证数据库数据的一致性

多个事务同时读取一个对象的时候，是不会有冲突的。同时读和写，或者同时写才会产生冲突。

因此为了提高数据库的并发性能，通常会定义两种锁：共享锁和排它锁。

共享锁：共享锁(S)表示对数据进行读操作。因此多个事务可以同时为一个对象加共享锁。
排他锁：排他锁(X)表示对数据进行写操作。如果一个事务对 对象加了排他锁，其他事务就不能再给它加任何锁了。

　

1. 查看岗位是teacher的员工姓名、年龄
select name,age from employee where post='teacher';

2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
select name,age from employee where post='teacher' and age>30;

3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
select name,age,salary from employee where salary between 9000 and 10000;

4. 查看岗位描述不为NULL的员工信息
select * from employee where post_comment is not null;

5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
select name,age,salary from employee where post='teacher' and salary in(10000,9000,30000); 

6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
select name,age,salary from employee where post='teacher' and salary not in(10000,9000,30000); 


7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
#select name,salary from employee where post='teacher' and name like 'jin_';
select name,salary from employee where post='teacher' and name like 'jin%';




1. 查询岗位名以及岗位包含的所有员工名字
select post,group_concat(name) from employee group by post;

2. 查询岗位名以及各岗位内包含的员工个数
select post,count(id) from employee group by post;

3. 查询公司内男员工和女员工的个数
select sex,count(id) from employee group by sex; 
4. 查询岗位名以及各岗位的平均薪资
select post,avg(salary) from employee group by post;

5. 查询岗位名以及各岗位的最高薪资
select post,max(salary) from employee group by post;

6. 查询岗位名以及各岗位的最低薪资
select post,min(salary) from employee group by post;


7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
select sex,avg(salary) from employee group by sex;


1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
select post,group_concat(name),count(id) from employee group by post having count(id)<2;
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary)>10000;
3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary)>10000 and avg(salary)<20000;



1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序
select * from employee order by age asc,hire_date desc;

2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
select post,avg(salary) from employee where avg(salary)>10000 group by post order by avg(salary) asc;#错误
select post,avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) asc;

3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
select post,avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) desc;


1. 分页显示，每页5条
select * from  employee limit 0,5;
select * from  employee limit 5,5;
select * from  employee limit 10,5;

1.以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,
即找出年龄大于25岁的员工以及员工所在的部门
select employee.name,department.name from employee 
    inner join department on employee dep_id=department.id where age>25;
    
2.以内连接的方式查询employee和department表，并且以age字段的升序方式显示
select employee.id,employee.name,employee.age,department.name from employee,department
    where employee dep_id=department.id  
    and age>25
    order by age asc;
    
1.查询平均年龄在25岁以上的部门名
select * from department 
    where id in 
        (select dep_id from employee group by dep_id having avg(age)>25);
    
2.查看技术部员工姓名
select name from employee
        where dep_id in
            (select id from department where name='技术');
            
3.查看不足1人的部门名
select name from department
    where id in 
        (select dep_id from employee group by dep_id having count(id)<=1);