Mysql单表查询-白红宇

Mysql单表查询

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发布时间：2019-06-11

本文共 21941 字，大约阅读时间需要 73 分钟。

本节重点：

　　单表查询

语法：

一、单表查询的语法   SELECT 字段1,字段2... FROM 表名                  WHERE 条件                  GROUP BY field                  HAVING 筛选                  ORDER BY field                  LIMIT 限制条数二、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fromwheregroup byhavingselectdistinctorder bylimit

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.将分组的结果进行having过滤

5.执行select

6.去重

7.将结果按条件排序：order by

8.限制结果的显示条数

创建公司员工表，表的字段和数据类型

company.employee    员工id          id                          int                      姓名            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,设置字段的约束条件create table employee(    id int primary key 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    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 || emp_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    |                || salart               | double(15,2)                  | YES  |         | NULL    |                || office              | int(11)                           | YES  |         | NULL    |                || depart_id        | int(11)                           | YES  |         | NULL    |                |+--------------+-----------------------+------+-----+---------+----------------+10 rows in set (0.08 sec)#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),('xiaomage','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);

创建员工表，并插入记录

（1）where 约束　

where子句中可以使用1.比较运算符：>、<、>=、<=、<>、!=2.between 80 and 100 ：值在80到100之间3.in(80,90,100)值是10或20或304.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符，_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

验证结果：

#1 :单条件查询mysql> select id,emp_name from employee where id > 5;+----+------------+| id | emp_name   |+----+------------+|  6 | jingliyang ||  7 | jinxin     ||  8 | xiaomage   ||  9 | 歪歪       || 10 | 丫丫       || 11 | 丁丁       || 12 | 星星       || 13 | 格格       || 14 | 张野       || 15 | 程咬金     || 16 | 程咬银     || 17 | 程咬铜     || 18 | 程咬铁     |#2 多条件查询mysql> select emp_name from employee where post='teacher' and salary>10000;+----------+| emp_name |+----------+| alex         || jinxin     |+----------+#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;#注意''是空字符串，不是null SELECT name,post_comment FROM employee WHERE post_comment=''; 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mysql>  SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;+------------+---------+| name       | salary  |+------------+---------+| yuanhao    | 3500.00 || jingliyang | 9000.00 |+------------+---------+2 rows in set (0.00 sec)mysql>  SELECT name,salary FROM employee  WHERE salary IN (3000,3500,4000,9000) ;+------------+---------+| name       | salary  |+------------+---------+| yuanhao    | 3500.00 || jingliyang | 9000.00 |+------------+---------+mysql>  SELECT name,salary FROM employee  WHERE salary NOT IN (3000,3500,4000,9000) ;+-----------+------------+| name      | salary     |+-----------+------------+| egon      |    7300.33 || alex      | 1000000.31 || wupeiqi   |    8300.00 || liwenzhou |    2100.00 || jinxin    |   30000.00 || xiaomage  |   10000.00 || 歪歪      |    3000.13 || 丫丫      |    2000.35 || 丁丁      |    1000.37 || 星星      |    3000.29 || 格格      |    4000.33 || 张野      |   10000.13 || 程咬金    |   20000.00 || 程咬银    |   19000.00 || 程咬铜    |   18000.00 || 程咬铁    |   17000.00 |+-----------+------------+16 rows in set (0.00 sec)#6:关键字LIKE模糊查询通配符’%’mysql> SELECT * FROM employee WHERE name LIKE 'jin%';+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+| 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 |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+2 rows in set (0.00 sec)通配符'_'mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';+-----+| age |+-----+|  78 |+-----+1 row in set (0.00 sec)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#对应的sql语句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%';

where约束

（2）group by 分组查询

#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的#2、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#3、为何要分组呢？    取每个部门的最高工资    取每个部门的员工数    取男人数和女人数小窍门：‘每’这个字后面的字段，就是我们分组的依据#4、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，需要借助于聚合函数

当执行以下sql语句的时候，是以post字段查询了组中的第一条数据，没有任何意义，因为我们现在想查出当前组的多条记录。

mysql> select * from employee 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                                 |              | 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如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BYmysql> set global sql_mode='ONLY_FULL_GROUP_BY';Query OK, 0 rows affected (0.00 sec)#查看MySQL 5.7默认的sql_mode如下：mysql> select @@global.sql_mode;+--------------------+| @@global.sql_mode  |+--------------------+| ONLY_FULL_GROUP_BY |+--------------------+1 row in set (0.00 sec)mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效Bye

继续验证通过group by分组之后，只能查看当前字段,如果想查看组内信息，需要借助于聚合函数

mysql> select * from emp group by post;# 报错ERROR 1054 (42S22): Unknown column 'post' in 'group statement'mysql>  select post from employee group by post;+-----------------------------------------+| post                                    |+-----------------------------------------+| operation                               || sale                                    || teacher                                 || 老男孩驻沙河办事处外交大使              |+-----------------------------------------+4 rows in set (0.00 sec)

（3）聚合函数

max()求最大值min()求最小值avg()求平均值sum() 求和count() 求总个数#强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组# 每个部门有多少个员工select post,count(id) from employee group by post;# 每个部门的最高薪水select post,max(salary) from employee group by post;# 每个部门的最低薪水select post,min(salary) from employee group by post;# 每个部门的平均薪水select post,avg(salary) from employee group by post;# 每个部门的所有薪水select post,sum(age) from employee group by post;

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

小练习

（4）HAVING过滤

HAVING与WHERE不一样的地方在于#！！！执行优先级从高到低：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发生在分组group by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：

验证：mysql> select * from employee where salary>1000000;+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+|  2 | alex | male |  78 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+1 row in set (0.00 sec)mysql> select * from employee having salary>1000000;ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause# 必须使用group by才能使用group_concat()函数，将所有的name值连接mysql> 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 'post' in 'field list'

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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

小练习答案：

# 题1：mysql> select post,group_concat(name),count(id) from employee group by post;+-----------------------------------------+-----------------------------------------------------------+-----------+| post                                    | group_concat(name)                                        | count(id) |+-----------------------------------------+-----------------------------------------------------------+-----------+| operation                               | 程咬铁,程咬铜,程咬银,程咬金,张野                          |         5 || sale                                    | 格格,星星,丁丁,丫丫,歪歪                                  |         5 || teacher                                 | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |         7 || 老男孩驻沙河办事处外交大使              | egon                                                      |         1 |+-----------------------------------------+-----------------------------------------------------------+-----------+4 rows in set (0.00 sec)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 |+-----------------------------------------+--------------------+-----------+1 row in set (0.00 sec)#题2：mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;+-----------+---------------+| post      | avg(salary)   |+-----------+---------------+| operation |  16800.026000 || teacher   | 151842.901429 |+-----------+---------------+2 rows in set (0.00 sec)#题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 row in set (0.00 sec)

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（5）order by 查询排序

按单列排序    SELECT * FROM employee ORDER BY age;    SELECT * FROM employee ORDER BY age ASC;    SELECT * FROM employee ORDER BY age DESC;按多列排序:先按照age升序排序，如果年纪相同，则按照id降序    SELECT * from employee        ORDER BY age ASC,        id DESC;

验证多列排序：SELECT * from employee ORDER BY age ASC,id DESC;mysql> SELECT * from employee ORDER BY age ASC,id DESC;+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| id | name       | sex    | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| 18 | 程咬铁     | female |  18 | 2014-05-12 | operation                               | NULL         |   17000.00 |    403 |         3 || 17 | 程咬铜     | male   |  18 | 2015-04-11 | operation                               | NULL         |   18000.00 |    403 |         3 || 16 | 程咬银     | female |  18 | 2013-03-11 | operation                               | NULL         |   19000.00 |    403 |         3 || 15 | 程咬金     | male   |  18 | 1997-03-12 | operation                               | NULL         |   20000.00 |    403 |         3 || 12 | 星星       | female |  18 | 2016-05-13 | sale                                    | NULL         |    3000.29 |    402 |         2 || 11 | 丁丁       | female |  18 | 2011-03-12 | sale                                    | NULL         |    1000.37 |    402 |         2 ||  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 ||  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    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 || 14 | 张野       | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 || 13 | 格格       | female |  28 | 2017-01-27 | sale                                    | NULL         |    4000.33 |    402 |         2 ||  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 || 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 ||  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 ||  8 | xiaomage   | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 ||  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    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 |+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+18 rows in set (0.01 sec)mysql>

验证多列排序

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1select * from employee ORDER BY age asc,hire_date desc;# 题目2mysql> 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 |+-----------+---------------+2 rows in set (0.00 sec)# 题目3mysql> 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 |+-----------+---------------+2 rows in set (0.00 sec)mysql>

小练习答案

（5）limit 限制查询的记录数:

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

小练习：

分页显示，每页5条

# 第1页数据  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)# 第2页数据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 | xiaomage   | 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)# 第3页数据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)

小练习答案