第四篇：表的查询

单表查询

1、前期表与数据准备

# 创建一张部门表
create table emp(
  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
);
 
 
# 插入记录
# 三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('tank','male',17,'20170301','张江第一帅形象代言部门',7300.33,401,1), # 以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('jason','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('大饼','male',18,'19000301','teacher',30000,401,1),
('sean','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

2、语法书写与执行顺序

语法顺序：

select id, name from emp where id > 3 and id < 6;

根据现实生活中图书馆管理员找书的过程: 先知道书在哪个位置，然后再判断要找的是什么书，最后再找书本中的第几页;

首先要知道数据在哪个位置，然后在判断要找的是什么数据，最后在从数据中筛选出需要的

执行顺序：

• 先from，相当于找到图书馆
• where， 相当于书在图书馆的位置
• select，相当于查找书中的某一页

执行顺序必须要清楚

对于查询数量大时，可以在表后面 加上\G，修改显示格式：

select * from emp\G  # 此处不要加分号
ariaDB [db1]> select * from emp\G
*************************** 1. row ***************************
          id: 1
        name: tank
         sex: male
         age: 17
   hire_date: 2017-03-01
        post: 张江第一帅形象代言部门
post_comment: NULL
      salary: 7300.33
      office: 401
   depart_id: 1
*************************** 2. row ***************************
          id: 2
        name: egon
         sex: male
         age: 78
   hire_date: 2015-03-02
        post: teacher
post_comment: NULL
      salary: 1000000.31
      office: 401
   depart_id: 1
*************************** 3. row ***************************
          id: 3
        name: kevin
         sex: male
         age: 81
   hire_date: 2013-03-05
        post: teacher
post_comment: NULL
      salary: 8300.00
      office: 401
   depart_id: 1
*************************** 4. row ***************************
          id: 4
        name: jason
         sex: male
         age: 73
   hire_date: 2014-07-01
        post: teacher
post_comment: NULL
      salary: 3500.00
      office: 401
   depart_id: 1

where约束条件

# 1.查询id大于等于3小于等于6的数据
select * from emp where id >= 3 and id <= 6;
# between: 两者之间
# and: 与
select * from emp where id between 3 and 6;
 
 
# 2.查询薪资是20000或者18000或者17000的数据
# or:  或者
select * from emp where salary=20000 or salary=18000 or salary=17000;
# in: 在什么里
select * from emp where salary in (20000, 18000, 17000);
# 3.查询员工姓名中包含o字母 的 员工姓名和薪资
# like: 模糊匹配
# %: 匹配0个或多个任意字符
# _: 匹配一个任意字符
select name, salary from emp where name like "%o%";
 
 
# 4.查询员工姓名是由四个字符组成的员工姓名与薪资
select name, salary from emp where name like "_____";
# char_length(字段): 获取字段长度
select name, salary from emp where char_length(name) = 4;
 
 
# 5.查询id小于3或者大于6的数据
select * from emp where id < 3 or id > 6;
select * from emp where id not between 3 and 6;
 
 
# 6.查询薪资不在20000，18000，17000范围的数据
select * from emp where salary not in (20000, 18000, 17000);
 
 
# 7.查询岗位描述为空的员工名与岗位名 
# 注意: 针对null不能用等号，只能用is
select name, post from emp where post_comment = null;
select name, post from emp where post_comment is null;
select name,post from emp where post_comment is not null;

group by

分组

严格模式：

严格模式下只能获取select后面查询的字段，无法获取其他字段信息，但是可以使用聚合函数来获取

聚合函数:

• max 求最大值
• min 求最小值
• sum 求和
• avg 求平均值
• count 统计个数

严格模式设置：

# 查看模式
MariaDB [db1]> show variables like "%mode%";
# 设置严格模式
MariaDB [db1]> set global sql_mode = "strict_trans_tables,only_full_group_by";
Query OK, 0 rows affected (0.00 sec)

设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据，
不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，因为不分组就是对单个元素信息的随意获取

严格模式下演示：

#  报错
MariaDB [db1]> select * from emp group by post;
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
# 报错
MariaDB [db1]> select id, name from emp group by post;
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
# 正常
MariaDB [db1]> select post from emp group by post;
+-----------------------------------+
| post                              |
+-----------------------------------+
| operation                         |
| sale                              |
| teacher                           |
| 张江第一帅形象代言部门            |
+-----------------------------------+
4 rows in set (0.00 sec)
 
MariaDB [db1]>

试题：

 
# 1.获取每个部门的最高工资
select post, max(salary) from emp group by post;
# as: 起别名; 给获取出来的数据字段名，设置别名
select post as '部门', max(salary) as '薪资' from emp group by post;
# 可简写, 但不推荐
select post '部门', max(salary) '薪资' from emp group by post;
 
# 每个部门的最低工资
select post, min(salary) from emp group by post;
 
# 每个部门的平均工资
select post, avg(salary) from emp group by post;
 
# 每个部门的工资总和
select post, sum(salary) from emp group by post;
 
# 每个部门的人数: count() 中传任意参数都没问题
select post, count(id) from emp group by post;
select post, count(post) from emp group by post;
 
 
# 2.查询分组之后的部门名称和每个部门下所有员工的姓名
# group_concat(name): 不仅可以获取分组后的某一个字段，并且可以对字符串进行拼接
select post, group_concat(name) from emp group by post;
# 给每个部门的员工名字前 + NB_
select post, group_concat('NB_', name) from emp group by post;
# 拼接部门员工名字+薪资
select post, group_concat(name, ":", salary) from emp group by post;
python
 
# 3.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法使用
select concat('Name: ', name) as '名字', concat('Sal: ', salary) as '薪资' from emp;
 
 
# 4.补充as语法 即可以给字段起别名也可以给表起
select emp.name as '名字', emp.salary as '薪资' from emp;
 
 
# 5.查询四则运算
# 求各部门所有员工的年薪
select name, salary * 12 as annual_salary from emp;

写查询语句的步骤: 先看需要查哪张表，然后看有没有什么限制条件, 再看需要根据什么分组，最后再看需要查看什么字段!

执行顺序:
from --> where --> group by --> select

注意: 聚合函数:
1、只能在group by后(执行顺序)使用
2、若查询语句没有group by，则默认整张表就是一个分组。

# 1. 查询岗位名以及各岗位内包含的员工个数
MariaDB [db1]> select post, count(name) from emp group by post;
+-----------------------------------+-------------+
| post                              | count(name) |
+-----------------------------------+-------------+
| operation                         |           5 |
| sale                              |           5 |
| teacher                           |           7 |
| 张江第一帅形象代言部门            |           1 |
+-----------------------------------+-------------+
4 rows in set (0.00 sec)
 
# 2.查询公司内男员工和女员工的个数
MariaDB [db1]> select sex, count(name) from emp group by sex;
+--------+-------------+
| sex    | count(name) |
+--------+-------------+
| male   |          10 |
| female |           8 |
+--------+-------------+
2 rows in set (0.00 sec)
 
# 3.查询每个部门内男员工和女员工的个数
MariaDB [db1]> select post, count(sex='male' or null) as male, count(sex='female' or null) as female from emp group by post;
+-----------------------------------+------+--------+
| post                              | male | female |
+-----------------------------------+------+--------+
| operation                         |    3 |      2 |
| sale                              |    0 |      5 |
| teacher                           |    6 |      1 |
| 张江第一帅形象代言部门            |    1 |      0 |
+-----------------------------------+------+--------+
4 rows in set (0.00 sec)
 
# 4.查询岗位名以及各岗位的平均薪资
MariaDB [db1]> select post, avg(salary) from emp group by post;
+-----------------------------------+---------------+
| post                              | avg(salary)   |
+-----------------------------------+---------------+
| operation                         |  16800.026000 |
| sale                              |   2600.294000 |
| teacher                           | 151842.901429 |
| 张江第一帅形象代言部门            |   7300.330000 |
+-----------------------------------+---------------+
4 rows in set (0.00 sec)
 
# 5.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
MariaDB [db1]> select sex, avg(salary) from emp group by sex;
+--------+---------------+
| sex    | avg(salary)   |
+--------+---------------+
| male   | 110920.077000 |
| female |   7250.183750 |
+--------+---------------+
2 rows in set (0.00 sec)
 
# 统计各部门年龄在30岁以上的员工平均工资
MariaDB [db1]> select post, avg(salary) from emp where age > 30 group by post;
+---------+---------------+
| post    | avg(salary)   |
+---------+---------------+
| sale    |   2500.240000 |
| teacher | 255450.077500 |
+---------+---------------+
2 rows in set (0.00 sec)
 

having

having: 过滤
1.having与where语法一样，只不过having需要在group by后使用;
2.where 不能使用聚合函数，但having可以;

• 执行顺序:
  • from
  • where
  • group by
  • having
  • select

MariaDB [db1]> select post, avg(salary) from emp where age > 30 group by post having avg(salary) > 100000;
+---------+---------------+
| post    | avg(salary)   |
+---------+---------------+
| teacher | 255450.077500 |
+---------+---------------+
1 row in set (0.00 sec)

distinct

distinct: 去重

注意: 查询的字段值必须是重复的才有效，只要有一个字段值是不重复的就没有效果。

执行顺序:
- from
- where
- group by
- having
- select
- distinct

select distinct id, post from emp;
select distinct post from emp;
select distinct age from emp;

order by

order by: 排序， 获取select获取的数据进行排序

# order by: 排序， 获取select获取的数据进行排序
 
# 1、根据薪资进行升序
select * from emp order by salary;  # 默认升序
select * from emp order by salary asc;  # 指定升序
select * from emp order by salary desc;  # 指定降序
 
# 2、根据年龄进行降序排列
select * from emp order by age desc;
 
# 3、先按照age升序，再按照salary降序
select * from emp order by age asc, salary desc;
 
# 4、统计 各部门(分组) 年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行升序序
select post, avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary);
 
执行顺序:
        - from
        - where
        - group by
        - having
        - select
        - order by  # 通过select 查出来的数据再进行排序

limit

limit: 限制结果返回数量

应用场景: 类似于博客园首页的数据展示，每一页有固定的数量;

# 1、从第一条开始，获取4条记录;
select * from emp limit 4;
 
# 2、limit可以有两个参数, 参数1:是限制的开始位置, 参数2:是从开始位置展示的条数;
select * from emp limit 0, 4;
select * from emp limit 4, 4;
 
# 3、查询工资最高的人的详细信息
select * from emp order by salary desc limit 1;
 
执行顺序:
        - from
        - where
        - group by
        - having
        - select
        - distinct
        - order by
        - limit

正则

# 在编程中，凡是看到reg开头的，基本上都是跟正则有关
select * from emp where name regexp '^程.*(金|银|铜|铁)$';

多表查询

• 多表查询
  • 关联查询
  • 子查询

创建表与插入数据准备

#建表
create table dep2(
id int,
name varchar(20)
);
 
create table emp2(
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 dep2 values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');
 
insert into emp2(name,sex,age,dep_id) values
('tank','male',17,200),
('egon','female',48,201),
('kevin','male',38,201),
('jason','female',28,202),
('owen','male',18,200),
('sean','female',18,204);
 
# 注意: 将拆分的表，再拼接到一起进行查询, 可以通过一张表查另一张表的数据;

关联查询

左表的一条记录与右表的一条记录都对应一遍称之为 --> "笛卡尔积"

将所有的数据都对应了一遍，虽然不合理但是其中有合理的数据，现在我们需要做的就是找出合理的数据

# 一 比较麻烦的表关联
1、查询员工以及所在部门的信息;
# 将两张表合并，并且根据id字段去判断
select * from emp2, dep2 where emp2.dep_id = dep2.id;
 
2、查询部门为技术部的员工及部门信息
select * from emp2, dep2 where emp2.dep_id = dep2.id and dep2.name = '技术';
 
 
# 二 将两张表关联到一起的操作，有专门对应的方法
1、inner join
# 1、内连接：只取两张表有对应关系的记录
select * from emp2 inner join dep2 on emp2.dep_id = dep2.id;
select * from emp2 inner join dep2 on emp2.dep_id = dep2.id and dep2.name = '技术';
 
2、left join
# 2、左连接: 在内连接的基础上保留左表没有对应关系的记录
select * from emp2 left join dep2 on emp2.dep_id = dep2.id;
 
3、right join
# 3、右连接: 在内连接的基础上保留右表没有对应关系的记录
select * from emp2 right join dep2 on emp2.dep_id = dep2.id;
 
4、union
# 4、全连接：在内连接的基础上保留左、右面表没有对应关系的的记录
select * from emp2 left join dep2 on emp2.dep_id = dep2.id
union
select * from emp2 right join dep2 on emp2.dep_id = dep2.id;

子查询

# 子查询就是将一个查询语句的结果用括号括起来，当做另一个查询语句的条件去用
 
# 1.查询部门是技术或者人力资源的员工信息
'''
先获取技术部和人力资源的id号，再去员工表里根据前面的id筛选出符合要求的员工信息;
'''
select * from emp2 where dep_id in (select id from dep2 where name='技术' or name='人力资源');
 
 
# 2.每个部门最新入职的员工 思路：先查每个部门最新入职的员工，再按部门对应上联表查询
# 查第一张emp表
select t1.id, t1.name, t1.hire_date, t1.post, t2.* from emp as t1
inner join
(select post, max(hire_date) as max_date from emp group by post) as t2
on t1.post = t2.post
where t1.hire_date = t2.max_date;
 
'''
as:
        - 可以给表起别名
        - 可以给查出来的虚拟表起别名
        - 可以给字段起别名
'''