1、准备表
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 #创建表
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
); #查看表结构
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 | |
-------------- ----------------------- ------ ----- --------- ---------------- #插入记录
#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,'','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,'','teacher',1000000.31,401,1),
('wupeiqi','male',81,'','teacher',8300,401,1),
('yuanhao','male',73,'','teacher',3500,401,1),
('liwenzhou','male',28,'','teacher',2100,401,1),
('jingliyang','female',18,'','teacher',9000,401,1),
('jinxin','male',18,'','teacher',30000,401,1),
('成龙','male',48,'','teacher',10000,401,1), ('歪歪','female',48,'','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'','sale',2000.35,402,2),
('丁丁','female',18,'','sale',1000.37,402,2),
('星星','female',18,'','sale',3000.29,402,2),
('格格','female',28,'','sale',4000.33,402,2), ('张野','male',28,'','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'','operation',20000,403,3),
('程咬银','female',18,'','operation',19000,403,3),
('程咬铜','male',18,'','operation',18000,403,3),
('程咬铁','female',18,'','operation',17000,403,3)
;
二 、查询语法
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
三、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，去文件/表中取出一条条记录
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.按照select后的字段得到一张新的虚拟表，如果有聚合函数，则将组内数据进行聚合
5.将4的结果过滤：having,如果有聚合函数也是先执行聚合再having过滤
6.查出结果：select
7.去重
8.将结果按条件排序：order by
9.限制结果的显示条数
四、简单查询
#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;
1 查出所有员工的名字，薪资,格式为
<名字:egon> <薪资:3000>
2 查出所有的岗位（去掉重复）
3 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 concat('<名字:',name,'> ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select name,salary*12 annual_salary from employee;
练习
五 、where约束
强调：where是一种约束条件，mysql会拿着where指定的条件去表中取数据，而having则是在取出数据后进行过滤
where字句中可以使用：
1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
4. like 'egon%'
pattern可以是%或_，
%表示任意多字符
_表示一个字符
5. 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
ps：
执行
update employee set post_comment='' where id=2;
再用上条查看，就会有结果了 #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__';
1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
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
4. 查看岗位描述不为null的员工信息
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
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
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪 select name,age from employee where post = 'teacher';
select name,age from employee where post='teacher' and age > 30;
select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select name,salary*12 from employee where post='teacher' and name like 'jin%';
练习
六 、分组查询:group by
大前提：可以按照任意字段分组,但分完组后，只能查看分组的那个字段，要想取的组内的其他字段信息，需要借助函数
单独使用group by关键字分组
select post from employee group by post;
注意：我们按照post字段分组，那么select查询的字段只能是post，想要获取组内的其他相关信息，需要借助函数 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; 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
强调：
如果我们用unique的字段作为分组的依据，则每一条记录自成一组，这种分组没有意义
多条记录之间的某个字段值相同，该字段通常用来作为分组的依据
参考链接：http://www.ywnds.com/?p=8184
#分组查询的常见问题：
mysql> select id,count from tt group by id;
error 1055 (42000): expression #2 of select list is not in group by clause and contains nonaggregated column 'test.tt.count' which is
not functionally dependent on columns in group by clause; this is incompatible with sql_mode=only_full_group_by #查看mysql 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
only_full_group_by,strict_trans_tables,no_zero_in_date,no_zero_date,error_for_division_by_zero,no_auto_create_user,no_engine_substitution #去掉only_full_group_by模式，如下操作：
mysql> set global sql_mode='strict_trans_tables,no_zero_in_date,no_zero_date,error_for_division_by_zero,no_auto_create_user,no_engine_substitution'; #！！！注意
only_full_group_by的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在only_full_group_by模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。
mysql 5.7默认only_full_group_by语义介绍
1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
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
#题1：分组
mysql> select post,group_concat(name) from employee group by post;
----------------------------------------- ---------------------------------------------------------
| post | group_concat(name) |
----------------------------------------- ---------------------------------------------------------
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙 |
| 老男孩驻沙河办事处外交大使 | egon |
----------------------------------------- --------------------------------------------------------- #题目2：
mysql> select post,count(id) from employee group by post;
----------------------------------------- -----------
| post | count(id) |
----------------------------------------- -----------
| operation | 5 |
| sale | 5 |
| teacher | 7 |
| 老男孩驻沙河办事处外交大使 | 1 |
----------------------------------------- ----------- #题目3：
mysql> select sex,count(id) from employee group by sex;
-------- -----------
| sex | count(id) |
-------- -----------
| male | 10 |
| female | 8 |
-------- ----------- #题目4：
mysql> select post,avg(salary) from employee group by post;
----------------------------------------- ---------------
| post | avg(salary) |
----------------------------------------- ---------------
| operation | 16800.026000 |
| sale | 2600.294000 |
| teacher | 151842.901429 |
| 老男孩驻沙河办事处外交大使 | 7300.330000 |
----------------------------------------- --------------- #题目5
mysql> select post,max(salary) from employee group by post;
----------------------------------------- -------------
| post | max(salary) |
----------------------------------------- -------------
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
----------------------------------------- ------------- #题目6
mysql> select post,min(salary) from employee group by post;
----------------------------------------- -------------
| post | min(salary) |
----------------------------------------- -------------
| operation | 10000.13 |
| sale | 1000.37 |
| teacher | 2100.00 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
----------------------------------------- ------------- #题目七
mysql> select sex,avg(salary) from employee group by sex;
-------- ---------------
| sex | avg(salary) |
-------- ---------------
| male | 110920.077000 |
| female | 7250.183750 |
-------- ---------------
练习
七、 使用聚合函数查询
先from找到表
再用where的条件约束去表中取出记录
然后进行分组group by，没有分组则默认一组
然后进行聚合
最后select出结果
示例：
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;
八 、having过滤
having与where在语法上是一样的
select * from employee where salary > 10000; select * from employee having salary > 10000;
#！！！执行优先级从高到低：where > group by > 聚合函数 > having
#1. where 是一个约束声明，使用where约束来自数据库的数据，where是在结果返回之前起作用的（先找到表，按照where的约束条件，从表(文件)中取出数据），where中不能使用聚合函数。 #2. having是一个过滤声明，是在查询返回结果集以后对查询结果进行的过滤操作（先找到表，按照where的约束条件，从表(文件)中取出数据，然后group by分组，如果没有group by则所有记录整体为一组，然后执行聚合函数，然后使用having对聚合的结果进行过滤），在having中可以使用聚合函数。 #3. having可以放到group by之后，而where只能放到group by之前 #4. 在查询过程中聚合语句(sum,min,max,avg,count)要比having子句优先执行。而where子句在查询过程中执行优先级高于聚合语句。
验证不同之处
having与where不一样的地方在于
#验证之前再次强调：执行优先级从高到低：where > group by > 聚合函数 > having
select count(id) from employee where salary > 10000; #正确，分析：where先执行，后执行聚合count(id),然后select出结果
select count(id) from employee having salary > 10000;#错误，分析：先执行聚合count(id)，后执行having过滤，无法对id进行salary>10000的过滤 #以上两条sql的顺序是
1:找到表employee--->用where过滤---->没有分组则默认一组执行聚合count(id)--->select执行查看组内id数目
2:找到表employee--->没有分组则默认一组执行聚合count(id)---->having 基于上一步聚合的结果（此时只有count(id)字段了）进行salary>10000的过滤，很明显，根本无法获取到salary字段
验证不同之处
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;
其他需要注意的问题
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
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
4. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资 #题1：
mysql> select post,group_concat(name),count(id) from employee group by post having count(id) < 2;
----------------------------------------- -------------------- -----------
| post | group_concat(name) | count(id) |
----------------------------------------- -------------------- -----------
| 老男孩驻沙河办事处外交大使 | egon | 1 |
----------------------------------------- -------------------- ----------- #题目2：
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
----------- ---------------
| post | avg(salary) |
----------- ---------------
| operation | 16800.026000 |
| teacher | 151842.901429 |
----------- --------------- #题目3：
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
----------- --------------
| post | avg(salary) |
----------- --------------
| operation | 16800.026000 |
----------- --------------
练习
九 、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
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
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 #题目1
mysql> select * from employee order by age asc,hire_date desc; #题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
----------- ---------------
| post | avg(salary) |
----------- ---------------
| operation | 16800.026000 |
| teacher | 151842.901429 |
----------- --------------- #题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
----------- ---------------
| post | avg(salary) |
----------- ---------------
| teacher | 151842.901429 |
| operation | 16800.026000 |
----------- ---------------
练习
十 、限制查询的记录数:limit
示例：
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
1. 分页显示，每页5条
mysql> select * from employee limit 0,5;
---- ----------- ------ ----- ------------ ----------------------------------------- -------------- ------------ -------- -----------
| id | 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 | | 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 |
---- ----------- ------ ----- ------------ ----------------------------------------- -------------- ------------ -------- -----------
5 rows in set (0.00 sec) mysql> select * from employee limit 5,5;
---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- -----------
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- -----------
| 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 |
---- ------------ -------- ----- ------------ --------- -------------- ---------- -------- -----------
5 rows in set (0.00 sec) mysql> select * from employee limit 10,5;
---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- -----------
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- -----------
| 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 |
---- ----------- -------- ----- ------------ ----------- -------------- ---------- -------- -----------
5 rows in set (0.00 sec)
练习
十一 、使用正则表达式查询
select * from employee where name regexp '^ale';
select * from employee where name regexp 'on$';
select * from employee where name regexp 'm{2}';
小结：对字符串匹配的方式
where name = 'egon';
where name like 'yua%';
where name regexp 'on$';
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]$';
练习