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: 对表中的数据进行过滤筛选
1.判断的符号
= > >= < <= != <> 不等于
2.拼接条件的关键字
and or not
3.查询范围区间 between
between 小值 and 大值 [小值,大值] 查询两者之间的这个范围所有数据
4.查询某个值在具体某个范围里 in
in(1,2,3,4)
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” 个数一共是2个字符,必须以a结尾,前面字符随意
like “a__” 个数一共是3个字符,必须以a开头,后面字符随意
# 单表练习
#创建表
create table employee(
id int not null unique auto_increment,
emp_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
);
#三个部门：教学，销售，运营
insert into employee(emp_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)
;
mysql> select * from employee;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 8 | 成龙 | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
(1)单条件的查询
查询部门是sale的所有员工姓名
select emp_name from employee where post = "sale";
(2)多条件的查询
部门是teacher,收入大于10000的所有数据
select * from employee where post = "teacher" and salary > 10000;
(3)between …and …
收入在1万~2万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1万~2万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 在查询时,需要使用is进行判断,不能用=
select * from employee where post_comment = null; # error
select * from employee where post_comment = ''; # error
select * from employee where post_comment is null;
select * from employee where post_comment is not null;
(5)in 在 … 之中
查询收入是 3000,4000,5000,8300所有的姓名和收入
select emp_name,salary from employee where salary in (3000,4000,5000,8300) # 推荐
select emp_name,salary from employee where salary = 3000 or salary = 4000 or salary = 5000 or salary = 8300;
not … in …
select emp_name,salary from employee where salary not in (3000,4000,5000,8300);
(6)模糊查询 like “%” “_”
# (1)匹配员工姓名 以on结尾的. "%" 通配符
select emp_name from employee where emp_name like "%on";
# (2)"_"通配符 限定字符长度使用_
select emp_name from employee where emp_name like "a_e_";
(7)concat (as起别名)
select concat("姓名:",emp_name,"工资:",salary) as ceshi from employee;
concat_ws(拼接的符号,参数1,参数2,参数2,参数3…)
select concat_ws(":",emp_name,salary) as ceshi from employee;
计算每个人的年薪 可以在mysql使用四则运算(+ - * /)
select concat_ws(":",emp_name,salary * 12) as ceshi from employee;
二.group by 字句 分组分类
group by 字段 对当前字段进行分类,by后面接什么字段,select 就搜什么字段
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) from employee group by post;
聚合函数
# 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;
1.查询部门名以及各部门的平均薪资 聚合函数 + 分组 配合使用
select post,avg(salary) from employee group by post;
2.查询部门名以及各部门的最高薪资
select post,max(salary) from employee group by post;
3.查询部门名以及各部门的最低薪资
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.查询部门名以及部门包含的所有员工名字
select post,group_concat(emp_name) from employee group by post;
可以group by 两个字段,搜索2个字段
select post,emp_name 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,分组之后过滤
找平均薪资大于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、岗位内包含员工名字、个数
select post,group_concat(emp_name),count(*) from employee group by post 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) > 10000 and avg(salary) < 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; # (默认是asc升序)
select * from employee order by age desc; # (倒序)
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,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);
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代表从第几条数据进行查询,0代表第一条,n代表的查询几条
select * from employee limit 0,5; # 从第一条数据开始搜,搜5条数据
select * from employee limit 5,5; # 从第六条数据开始搜
# 只搜索一条数据
select * from employee limit 1;
# 只搜索3条数据
select * from employee limit 3;
# 只搜索这个表中最后一条数据
select * from employee order by id desc limit 1;
六.(了解)可以使用正则表达式查询数据(不推荐,效率不高)
select * from employee where emp_name regexp ".*on$"; # .*? 这个?号 mysql 不识别
select * from employee where emp_name regexp "程";
select * from employee where emp_name regexp "程.*金";
part2 多表查询
内连接:
内连接: (inner join) -> 两表或者多表满足条件的所有数据查询出来(两表之间的共有数据)
# 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 + 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
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,左表没有的补null
select * from employee right join department on employee.dep_id = department.id;
3.全连接
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;
part3 子查询
子查询:嵌套查询
(1)sql语句当中又嵌套了另外一条sql语句,用()包起来,表达一个整体
(2)一般应用在from 字符后面(表达一张表),where 字句后面(表达一个条件)
(3)查询速度从快到慢: 单表查询 -> 联表速度 -> 子查询
# 多表练习:
#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)
;
department
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+
employee
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 | 18
| 2 | alex | female | 48 | 201 | 43
| 3 | wupeiqi | male | 38 | 201 | 43
| 4 | yuanhao | female | 28 | 202 | 28
| 5 | liwenzhou | male | 18 | 200 | 18
| 6 | jingliyang | female | 18 | 204 | 18
+----+------------+--------+------+--------+
一.查询员工都在哪些部门(200 201 202 204)
select dep_id from employee group by dep_id
二.把不在这些部门的数据找出来
select id from department where id not in (200,201,202,204)
三.综合拼接
select id,name from department where id not in (select dep_id from employee group by dep_id);
四.查询大于平均年龄的员工名与年龄
1.如果平均年龄是25;
select name,age from employee where age > 25
2.计算平均年龄
select avg(age) from employee
3.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) as avg_age from employee group by dep_id
2.把查询各部门的平均年龄和employee做联表,变成更大的表方便后期做单表查询
select
*
from
employee as t1 inner join (1号数据) 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.做最后的条件筛选
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
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id | max_date
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 | 2038-1-1
| 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 8 | 成龙 | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
1.找每个部门最大的入职时间
select post,max(hire_date) as max_date from employee group by post;
2.把子查询搜索出来的数据和employee联合成一张更大的表,做一次单表查询
select
*
from
employee as t1 inner join (1号数据) as t2 on t1.post = t2.post
where
t1.hire_date = t2,max_date
综合拼接
select
t1.emp_name,t1.hire_date
from
employee as t1 inner join (select post,max(hire_date) as max_date from employee group by post) as t2 on t1.post = t2.post
where
t1.hire_date = t2.max_date
七.带EXISTS关键字的子查询
EXISTS 关键字,表达存在
如果内层sql 能够查询到数据,返回True,外层sql执行查询语句
如果内层sql 不能够查询到数据,返回False,外层sql不执行查询语句
select * from employee where exists (select * from employee where id = 100);
子查询总结:
子查询可以单独作为一个临时数据表,临时数据,临时字段
一般用在 from where select 字句后面
可以通过查询出来的临时数据和另外的表做一次联表,变成更大的表,然后做单表查询,以得到相要的结果
额外 distinct 去重[尝试操作]
select distinct depart_id from employee;