单表查询，多表查询，子查询---day37

1.单表查询

# ### part1 单表查询
# sql 查询语句的完整语法
'''select..from..where..group by..having..order by..limit..'''

#一.where 条件的使用
'''功能：对表中的数据进行筛选和过滤'''
'''
语法：
    1.判断的符号
    = > >= < <= != <>  不等于
    2.拼接条件的关键字
    and or not
    3.查询的区间范围值 between
    between 小值 and 大值 [小值,大值]查询两者之间这个范围内所有数据
    4.查询具体某个值的范围  in
    in(1,2,3) 指定范围
    5.模糊查询 like "%" "_" 通配符
        like "%a" 匹配以a结尾的任意长度的字符串
        like "a%" 匹配以a开头的任意长度的字符串
        like "%a%"匹配含有a字母的任意长度的字符串
        like "_a" 个数一共2个字符，必须以a结尾，前面的字符随意
        like "a__"个数一共3个字符,必须以a开头，后面的字符随意
'''

#(1) 单条件的查询
#查询部分是sale的所有员工姓名
select emp_name from employee where post = "sale";

#(2)多条件的查询
#部分是teacher，收入大于10000的所有数据
select * from employee where post = "teacher" and salary >10000;

#(3)关键字 between..and..
#收入在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary between 10000 and 20000;
#收入不在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary not between 10000 and 20000;

#(4) null 关键字 在查询的时候，要用is进行判定，不要用=
#查询post_comment 是空的所有数据
select * from employee where post_comment is null;
select * from employee where post_comment is not null;

#(5)关键字 in 在..之中查询
#查询收入是3000,4000,5000,8300所有员工的姓名和收入
select emp_name,salary from employee where salary in(3000,4000,5000,8300);
#不在  not in
select emp_name,salary from employee where salary not in (3000,4000,5000,8300);

#(6)模糊查询 like "%" '_'通配符
#(1)"%"通配符 以on结尾的员工名搜一下
select emp_name,age,post from employee where emp_name like '%on';
#(2) "_"通配符 可以限定具体的长度
select emp_name,age,post from employee where emp_name like "a_e_";

#(7) concat( as 起别名)
select concat("姓名:",emp_name,"工资:",salary) as aa from employee;
#concat_ws(拼接的符号,参数1,参数2,参数3,...)
select concat_ws(":",emp_name,salary) as bb from employee;
#计算年薪 可以在mysql中使用四则运算符(+ - * /)
select concat_ws(":",emp_name,salary*12) as  cc from employee;

#二.group by 子句 分组分类
'''group by 字段 对数据进行分类,by后面接什么字段，select就搜索什么字段'''
select sex from employee group by sex;
select post from employee group by post;
#group_concat 按照分类的形式进行字段的拼接
select group_concat(emp_name),post from employee where id >5 group by post;

#配合聚合函数使用
#count 统计总数 * 所有
select count(*) from employee;
#max 统计最大值
select max(salary) from employee;
#min 统计最小值
select min(salary) from employee;
#avg 统计平均值
select avg(salary) from employee;
#sum 统计总和
select sum(salary) from employee;

#一般情况下 分组 + 聚合函数 配合使用
#1。查询部分名以及各部分的平均薪资
select post,avg(salary) from employee group by post;
#2.查询部分名以及个部分的最高薪资
select post,max(salary) from employee group by post;
#3.查询部门名以及个部分的最低薪资
select post,min(salary) from employee group by post;
#4.查询公司内男员工和女员工的个数
select sex,count(*) from employee group by sex;
#5.查询部分名以及部门包含的所有员工名字
select group_concat(emp_name) from employee group by post;
#可以group by 两个字段，即可搜索两个字段
select emp_name,post from employee group by post,emp_name;

#三.having 数据在分类分组之后，进行二次数据过滤，一般是配合group by使用，分组之后再过滤
#找出各部门平均薪资，并且大于10000以上的所有部门
select post,avg(salary) from employee group by post having avg(salary) > 10000;

#1.查询各岗位内包含的员工个数小于2的岗位名、查询岗位内包含员工名字、个数
select post,group_concat(emp_name),count(*) from employee group by having count(*) < 2;
#2.查询各岗位平均薪资小于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) between 10000 and 20000;

#四.order by 排序，按照什么字段进行排序
#asc  升序：从小到大(默认)
#desc 降序：从大到小
select * from employee order by age;(asc 可以不写)
select * from employee order by age desc;

#1.查询所欲员工信息，先按照age升序排序如果age相同则按照hire_date降序排序
select emp_name,age,hire_date,post from employee order by age,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) 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;

#五.limit 限制查询条数(数据分页)
'''limit m,n m代表从第几条数据查询,n代表查询几条，m=0 代表的是第一条'''
select * from employee limit 0,5; #从第一条数据开始搜，搜5条
select * from employee limit 5,5; #从第六条数据开始搜，搜5条
#只查询一条数据
select * from employee limit 1;
#找数据库当中最后一条数据
select * from employee order by id limit 1;
#找数据库当中最后三条数据
select * from employee order by id limit 3;

#六(了解) 可以使用正则表达式查询数据(不推荐使用,效率不高)
select * from employee where emp_name regexp ".*?on$"; #.*?不识别
select * from employee where employee regexp "^程";
select * from employee where employee regexp "^程.*金";

2.多表查询

# ### part2 多表查询
#内连接:(内联查询 inner join) -> 两表或者多表满足条件的所有数据查询出来(两表之间共有的数据)
'''
#两表查询
select 字段 from 表1 inner join 表2 on 必要的关联条件
#多表查询
select 字段 from 表1 inner join 表2 on 必要的关联条件1 inner join 表3 on 必要的关联条件2...
'''
#基本语法 inner join on .. on 后面接必要的关联条件
select * from employee inner join department on employee.dep_id = department.id;
#用as 起别名(推荐)
select * from employee as e inner join department as d on e.dep_id = d.id;
#as 可以省略掉
select * from  employee e inner join department d on e.dep_id = d.id;

#where 默认实现的就是内联查询的效果
select * from employee,department where employee.dep_id = department.id;
select * from employee as e,department as d where e.dep_id = d.id;

#外连接
#(1)左连接(左联查询left join)以左表为主，右表为辅，完整查询左表所有数据，右表没有的数据补null
select * from employee left join department on employee.dep_id = department.id;
#(2)右连接(右联查询right join)以右表为主，左表为辅，完整查询右表所有数据，左表没有的数据补null
select * from employee right join department on employee.dep_id = department.id;
#(3)全连接(全连接union) 所有的数据都合并起来
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;

3.子查询

# ###part3 子查询
'''
子查询：嵌套查询
(1)sql语句当中有嵌套了另外一条sql语句，用括号()包起来，表达一个整体
(2)一般应用在from子句后面表达一张表，where子句后面表达一个条件
(3)查询速度从快到慢:单表查询 -> 联表查询 -> 子查询
'''

#一.找出平均年龄大于25岁以上的部分
#(1)普通where写法
select d.id,d.name from employee as e,department as d
where e.dep_id = d.id group by d.id,d.name having avg(e.age) > 25;
#(2) inner join
select d.id,d.name from employee as e inner join department as d
on e.dep_id = d.id group by d.id,d.name having avg(e.age) > 25;
#(3)子查询
#1.先选出平均年龄大于25的部分id
select dep_id from employee group by dep_id having avg(age) > 25;
#2.通过部门id，找部门的名字
select name from department where id in(201,202);
#3.综合拼接
select id,name from department where id in (select name from department where id in(201,202))

#二。查看技术部门员工姓名
#(1) 普通where 查询
select e.name,d.name from department as d,employee as e
where d.id = e.dep_id and d.name = '技术';
#(2) inner join 写法
select e.name,d.name from department as d inner join employee as e
on d.id = e.dep_id where d.name = "技术";
#(3)子查询
#1.找技术部门对应id
select id from department where name = "技术";
#2.通过id找员工姓名
select name from employee where dep_id = 200;
#3.综合拼接
select name,dep_id from  employee where dep_id = (select id from department where name = "技术");

#三.查看哪个部门没有员工
#联表写法(把null的数据露出来，员工的部门dep_id 为null，代表这个部门没人)
select d.id,d.name from department as d left join employee as e
on d.id = e.dep_id where e.dep_id is null;
#子查询
#1.先查询，员工在哪些部门(所有员工的分类分别是200 201 202 204)
select dep_id from employee group by dep_id
#2.把不在部门的数据找出来
select id from department where id not in(200,201,202,204);
#综合拼接
select id,name from department where id not in(select id from department where id not in(200,201,202,204));

#四.查询大于平均年龄的员工名与年龄
#假设平均年龄20岁
select name,age from employee where age >20
#找平均年龄
select avg(age) from employee;
#综合拼接
select name,age from employee where age>(select avg(age) from employee);

#五。把大于其本部门平均年龄的员工名和姓名查出来
#1.先计算各部门平均年龄是多少
select dep_id,avg(age) from  employee group by dep_id;
#2.把查询的各部门的平均年龄和过去的employee联表，变成更大的表，方便做一次单查询
select * from employee as t1 inner join (1号查询到的数据) as t2 on t1.dep_id = t2.dep_id;
#3.综合拼接
select * from employee as t1 inner join
(select  dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id;
#4.做最后的数据筛选age >平均年龄
select * from employee as t1 inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
on t1.dep_id = t2.dep_id where t1.age > t2.avg_age;

# 六.查询每个部门最新入职的那位员工  # 利用上一套数据表进行查询;
#1.找每个部门最大的入职时间
select post,max(hire_date) as max_data from employee group by post;
#2.把子查询搜出来的数据和employee联合成一张更大的表，做一次单标查询
select t1.emp_name,t1.hire_date from employee t1 inner join
(1号查出来的数据) as t2 on t1.post = t2.post where t1.hire_date = t2.max_data;
#3.综合拼接
select t1.emp_name,t1.hire_date from employee t1 inner join
(select post,max(hire_date) as max_data from employee group by post) as t2 on t1.post = t2.post where t1.hire_date = t2.max_data;

#七.dai EXISTS关键字的子查询
'''
exists 关键字，表达存在
如果内层sql 能够查到数据，返回True，外层sql执行查询语句
如果内层sql不能查到数据，返回False，外层sql不执行查询语句
'''
select * from employee where exists(select * from employee where id = 1);#能
select * from employee where exists(select * from employee where id = 100);#不能

'''
子查询总结：
    子查询可以单独作为一个临时数据，临时的表，临时的字段
    一般用在from where select 子句后面
    可以通过查询出来的临时数据和另外的表联合，变成一张更大的表，在做单表查询查到想要的数据
'''

4.补充

# ### 补充
#关于约束的添加和删除
#1.添加/删除 约束 not null
    #alter table t1 modify id int not null;
    alter table t1 modify id int;

#2.添加/删除 unique 唯一索引
    #alter table 表名 add unique(id)
    alter table t1 add unique(id)
    alter table t1 drop index id

#3.添加/删除 primary key
    #alter table 表名 add primary key(id);
    alter table t1 add primary key(id);
    alter table t1 add primary key;

#4.添加/删除 foreign key 外键(先通过desc 表 找到外键名字，然后再删)
    alter table student1 drop foreign key student1_ibfk_1; #删除
    alter table student1 add foreign key(classid) references class1(id);#添加