week7 day3 记录相关操作之单表查询

  • 1.1 单表查询的用法
  • 1.2 关键字的执行优先级（重点）
  • 1.3 简单查询
  • 1.4 WHERE约束
  • 1.5 分组查询GROUP BY
  • 1.6 HAVING过滤
  • 1.7 查询排序ORDER BY
  • 1.8 限制查询的记录数LIMIT
  • 1.9 使用正则表达式查询

1.1 单表查询的用法

select 字段1,字段2,字段3...
from 表名
where 条件
group by field
having 筛选
order by field
limit 限制条数

1.2 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，则整体为一组
  4. 将分组结果进行having过滤
  5. 执行select
  6. 去重
  7. 将结果按照条件排序order by
  8. 限制结果的显示条数

1.3 简单查询

# 准备表和记录
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)
;#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk准备表和记录

# 简单查询
select * from employee;
select name,salary from employee;# 避免重复
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 Annul_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;结合case语句
select(casewhen name = 'egon' thennamewhen name = 'alex' thenconcat(name,'_BIGsb')elseconcat(NAME,'SB')end)as new_name
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;

1.4 WHERE约束

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可以实%或_，
    %表示任意多字符
    _表示一个字符
  5. 逻辑运算符：在多个条件直接可与使用逻辑运算符 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>1000;# 3 关键字between and
select name,salary from employee
where salary between 1000 and 20000;select name,salary from employee
where salary not between 1000 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''是空字符串而不是nullps：执行update employee set post_comment='' where id =2;再用上条查看，就知道结果了# 5 关键字in集合查询
select name,salary from employee
where salary=3000 or salary=4000 or salary=5000;select name,salary from employee
where salary in (3000,4000,5000);select name,salary from employee
where salary not in (3000,4000,5000);# 6 关键字like模糊查询通配符 %select name ,salary from employeewhere name like 'eg%';通配符 _select name,salary from employeewhere name like 'eg_';

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
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%';

1.5 分组查询GROUP BY

  1. 什么是分组？为什么要分组？
    一. 首先明确一点：分组发生在where之后，即分组是基于where之后，即分组是基于where之后得到的记录而进行的。
    二. 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。
    三. 为何要分组呢？取每个部门最高工资 / 取每个部门的员工数 / 取男人和女人数
    小窍门：需求中的 “每”、“平均”、“最高”、“最低” 后面的字段，就是我们分组的依据
    四. 大前提：可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助聚合函数，但使用聚合函数只能查看数字类型字段。想查看字符串类型字段用group_concat(xxx)

  2. ONLY_FULL_GROUP_BY（修改一下配置）

    #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#！！！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。#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
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';！！！SQL_MODE设置！！！

    mysql> select @@global.sql_mode;
+-------------------+
| @@global.sql_mode |
+-------------------+
|                   |
+-------------------+
1 row in set (0.00 sec)mysql> select * from emp group by post;
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| id | name | sex    | age | hire_date  | post                       | post_comment | salary     | office | depart_id |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
| 14 | 张野 | male   |  28 | 2016-03-11 | operation                  | NULL         |   10000.13 |    403 |         3 |
|  9 | 歪歪 | female |  48 | 2015-03-11 | sale                       | NULL         |    3000.13 |    402 |         2 |
|  2 | alex | male   |  78 | 2015-03-02 | teacher                    | NULL         | 1000000.31 |    401 |         1 |
|  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 |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)mysql> quit #设置成功后，一定要退出，然后重新登录方可生效
Byemysql> use db1;
Database changed
mysql> select * from emp group by post; #报错
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
mysql> select post,count(id) from emp group by post; #只能查看分组依据和使用聚合函数
+----------------------------+-----------+
| post                       | count(id) |
+----------------------------+-----------+
| operation                  |         5 |
| sale                       |         5 |
| teacher                    |         7 |
| 老男孩驻沙河办事处外交大使      |         1 |
+----------------------------+-----------+
4 rows in set (0.00 sec)

  3. GROUP BY

    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，想要获得组内的其他相关值信息，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 enployee group by post; # 按照岗位分组select post,group_concat(name) as emp_menbers 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

  4. 聚合函数

    # 强调：聚合函数聚合的是组内的内容，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;

  5. 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
#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 |
+--------+---------------+

1.6 HAVING过滤

HAVING和WHERE不一样的地方在于
执行顺序从高到低：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group by分组之后，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
# 验证
mysql> select @@sql_mode;
+--------------------+
| @@sql_mode         |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in set (0.00 sec)mysql> select * from emp where salary > 100000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | alex | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
1 row in set (0.00 sec)mysql> select * from emp having salary > 100000;
ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clausemysql> select post,group_concat(name) from emp group by post having salary > 10000;#错误，分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000;
+-----------+-------------------------------------------------------+
| post | group_concat(name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
+-----------+-------------------------------------------------------+
2 rows in set (0.00 sec)验证

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
  2. 查询各岗位平均薪资大于10000的岗位名、平均工资
  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
#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 |
+-----------+--------------+

1.7 查询排序ORDER BY

按单列排序
select * from employee order by salary; 不写默认是升序
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; # 年龄升序，薪资降序

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
#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 |
+-----------+---------------+

1.8 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，每页五条
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)

1.9 使用正则表达式查询

select * from employee where name regexp '^ale'; ^：以xxx开头
select * from employee where name regexp 'on$'; $：以xxx结尾
select * from employee where name regexp 'm{2}';小结：对字符串匹配的方式
where name = 'egon';
where name like 'egon%'/'_egon';
where name regexp 'on$';

小练习：

  1. 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]$';

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