Mysql ==》 单表查询
主要内容:
1.查询语法
2.关键字的执行优先级
3.简单查询
4.单条件查询
5.where 约束
6.分组查询: group by
7.使用聚合函数查询
8.having 过滤
9.查询排序:order by
10限制查询的记录数:limit
11.使用正则表达式查询
先建数据库(文件夹)、后表(文件)带字段,后记录(内容)。
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,'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)
一:查询语法:
select 字段1,字段2... from 表名where 条件group by fieldhaving 筛选order by fieldlimit 限制条数
二:关键字的执行优先级(重点)
重点:关键字的优先级执行顺序
1.from
2.where
3.group by
4.having
5.select
6.distinct
7.order by
8.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
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
三: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DISTINCTselect 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;
小练习:
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;
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四:where 约束
强调: where是一种约束条件,mysql会拿着where指定的条件取表中取数据,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 'aray%'
patterm 可以是 %或_,
%表示任意字符, _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.关键字查询:between andselect name,salary from employee where salary between 10000 and 20000;select name,salary from employee where sal''ary not between 10000 and 20000;4.关键字查询:is null(判断某个字段是否为null 不能用等号们需要用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。ps:执行update employee set post_comment='' where id=2;再用上条查看,就会有结果了。5.关键字查询: in select name,salary from employee where salary=3000 or salary=3500 or salary=4000;select name,salary from employee where salary in (3000,3500,4000);select name,salary from employee where salary not in (3000,3500,4000);6.关键字查询:like通配符 '%'select * from employee where name like 'ar%';通配符: '_'select * from employee where name like 'ar__';
小练习:
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
1. select name,age from employee where post ='teacher';2. select name,age from employee where post='teacher' and age>30; '' 3. select name,age,salary from employee where post='teacher' and salary between 9000 adn 10000;4. select * from employee where post_comment is not null;5. select name,age,salary from employee where post='teacher' and salary in(10000,9000,30000);6. select name,age,salary from employee where post='teacher' and salary not in(10000,9000,30000);7. select name,salary from empolyee where post='teacher' and name like 'jin%';
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五: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,要获取组内的其他信息,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; #按岗位分组,并查看成员名。select post,group concat(name) as emp_members from employee group by post;group ty 与聚合函数一起使用select post,count(id) as count from employee group by post; #按岗位分组,并查看每个组有多少人
强调:
如果我们用unique 的字段做为 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。
小练习:
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
1. select post,group_concat(name) from employee group by post;2. select post,count(id) from employee group by post;3. select sex,count(id) from employee group by sex;4. select post,avg(salary) from employee group by post;5. select post,max(salary) from employee group by post;6. select post,min(salary) from employee group by post;7. select sex,avg(salary) from employee group by sex;
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六: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 empoloyee where depart_id=1;select max(salary) from emploee;select min(salary) from employee;select avg(salary) from employee;select sum(salary) from employee;select sum(salary) from employee where depart_id=3;
七: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;
不同点:
1.执行优先级:从高到低。where > group by > 聚合函数 > having 2. where 是一个约束条件,是在结果返回之前起作用的。(先找到表,按where的约束条件,取出数据)having 是一个过滤条件,是在结果返回以后起作用的。(取出数据后,在group by 分组,如果没有group by 则所有的记录为一组,在执行聚合函数,然后使用having 对聚合的结果进行过滤),在having中可以用聚合函数,where则不能。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有:(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:
#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
其他需要注意的问题:
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;
小练习:
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
1. select post,group_concat(name),count(id) from empoloyee group by post having count(id) < 2;2. select post,avg(salary) from employee group by post having avg(salary) > 10000;3. select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) < 20000;
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八:查询排序: order by
按单列排序:select * from employee oeder by salary;select * from employee order by asc;select * from employee order by desc;按多列排序:先按age排序,如果年纪相同,则按照薪资排序select * from employee order by age,salary desc;
小练习:
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
1. select * from employee order by age asc,hire_data desc;2. select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;3. select post,avg(salary) from employee group by post having avg(salary) >10000 order bu avg(salary) desc;
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九: 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 DESCLIMIT 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 DESCLIMIT 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
小练习:
1. 分页显示,每页5条
#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 | +-----------+---------------+回到顶部 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 DESCLIMIT 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 DESCLIMIT 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 小练习: 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)
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十:使用正则表达式查询
select * from employee where name regexp '^ale';select * from employee where name regexp 'on$';select * from employee where name regexp 'm{2}';小结:对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';
小练习:
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]$';
转载于:https://www.cnblogs.com/zhongbokun/p/7501484.html
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