【MySQL】单表查询

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目录

where 约束

group by 分组查询

聚合函数

having 过滤

order by 查询排序

limit 限制查询的记录数

  1. # 语法
  2. select 字段1, 字段2 ... from 表名
  3.     where 条件
  4.     group by field
  5.     having 筛选
  6.     order by field
  7.     limit 限制条数

重点在于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. 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

  1. company.employee
  2.     员工id          id                          int                  
  3.     姓名            name                        varchar                                                             
  4.     性别            sex                         enum                                                                  
  5.     年龄            age                         int
  6.     入职日期         hire_date                   date
  7.     岗位            post                        varchar
  8.     职位描述         post_comment         　　　　varchar
  9.     薪水            salary                　　　 double
  10.     办公室           office                     int
  11.     部门编号         depart_id                   int
  1. # 创建员工表
  2. mysql> create table employee(
  3.     -> id int primary key auto_increment,
  4.     -> name varchar(20) not null,
  5.     -> sex enum('male', 'female') not null default 'male',
  6.     -> age int(3) unsigned not null default 28,
  7.     -> hire_date date not null,
  8.     -> post varchar(50),
  9.     -> post_comment varchar(100),
  10.     -> salary double(15, 2),
  11.     -> office int,
  12.     -> depart_id int
  13.     -> );
  14. Query OK, 0 rows affected (0.04 sec)
  15.  
  16. mysql> desc employee;
  17. +--------------+-----------------------+------+-----+---------+----------------+
  18. | Field        | Type                  | Null | Key | Default | Extra          |
  19. +--------------+-----------------------+------+-----+---------+----------------+
  20. | id           | int(11)               | NO   | PRI | NULL    | auto_increment |
  21. | name         | varchar(20)           | NO   |     | NULL    |                |
  22. | sex          | enum('male','female') | NO   |     | male    |                |
  23. | age          | int(3) unsigned       | NO   |     | 28      |                |
  24. | hire_date    | date                  | NO   |     | NULL    |                |
  25. | post         | varchar(50)           | YES  |     | NULL    |                |
  26. | post_comment | varchar(100)          | YES  |     | NULL    |                |
  27. | salary       | double(15,2)          | YES  |     | NULL    |                |
  28. | office       | int(11)               | YES  |     | NULL    |                |
  29. | depart_id    | int(11)               | YES  |     | NULL    |                |
  30. +--------------+-----------------------+------+-----+---------+----------------+
  31. 10 rows in set (0.00 sec)
  32.  
  33.  
  34. # 插入记录
  35. # 三个部门：教学，销售，运营
  36. mysql> insert into employee(name ,sex,age,hire_date,post,salary,office,depart_id) values
  37.     -> ('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1),
  38.     -> # 以下是教学部
  39.     -> ('alex','male',78,'20150302','teacher',1000000.31,401,1),
  40.     -> ('wupeiqi','male',81,'20130305','teacher',8300,401,1),
  41.     -> ('yuanhao','male',73,'20140701','teacher',3500,401,1),
  42.     -> ('liwenzhou','male',28,'20121101','teacher',2100,401,1),
  43.     -> ('jingliyang','female',18,'20110211','teacher',9000,401,1),
  44.     -> ('jinxin','male',18,'19000301','teacher',30000,401,1),
  45.     -> ('xiaomage','male',48,'20101111','teacher',10000,401,1),
  46.     -> # 以下是销售部
  47.     -> ('歪歪','female',48,'20150311','sale',3000.13,402,2),
  48.     -> ('丫丫','female',38,'20101101','sale',2000.35,402,2),
  49.     -> ('丁丁','female',18,'20110312','sale',1000.37,402,2),
  50.     -> ('星星','female',18,'20160513','sale',3000.29,402,2),
  51.     -> ('格格','female',28,'20170127','sale',4000.33,402,2),
  52.     -> # 以下是运营部
  53.     -> ('张野','male',28,'20160311','operation',10000.13,403,3),
  54.     -> ('程咬金','male',18,'19970312','operation',20000,403,3),
  55.     -> ('程咬银','female',18,'20130311','operation',19000,403,3),
  56.     -> ('程咬铜','male',18,'20150411','operation',18000,403,3),
  57.     -> ('程咬铁','female',18,'20140512','operation',17000,403,3)
  58.     -> ;

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80或90或100
  4. like 'z%' 或 like 'zy_'        # %表示任意多字符，_表示任意一个字符
  5. and or not        # 逻辑运算符，在多个条件可以直接使用逻辑运算符

实测：

  1. # 单条件查询
  2. mysql> select id,name from employee where id < 5;
  3. +----+---------+
  4. | id | name    |
  5. +----+---------+
  6. |  1 | egon    |
  7. |  2 | alex    |
  8. |  3 | wupeiqi |
  9. |  4 | yuanhao |
  10. +----+---------+
  11. 4 rows in set (0.00 sec)
  12.  
  13. # 多条件查询
  14. mysql> select name from employee where post='teacher' and salary>10000;
  15. +--------+
  16. | name   |
  17. +--------+
  18. | alex   |
  19. | jinxin |
  20. +--------+
  21. 2 rows in set (0.00 sec)
  22.  
  23. # 关键字 between and
  24. mysql> select name,salary from employee where salary between 10000 and 20000;
  25. +-----------+----------+
  26. | name      | salary   |
  27. +-----------+----------+
  28. | xiaomage  | 10000.00 |
  29. | 张野      | 10000.13 |
  30. | 程咬金    | 20000.00 |
  31. | 程咬银    | 19000.00 |
  32. | 程咬铜    | 18000.00 |
  33. | 程咬铁    | 17000.00 |
  34. +-----------+----------+
  35. 6 rows in set (0.00 sec)
  36.  
  37.  
  38. # 注意：''是空字符串，不是null
  39. mysql> select name,post_comment from employee where post_comment='';
  40. Empty set (0.00 sec)
  41.  
  42. # 执行如下代码后再此查询
  43. mysql> update employee set post_comment='' where id=2;
  44. Query OK, 1 row affected (0.01 sec)
  45. Rows matched: 1  Changed: 1  Warnings: 0
  46.  
  47. mysql> select name,post_comment from employee where post_comment='';
  48. +------+--------------+
  49. | name | post_comment |
  50. +------+--------------+
  51. | alex |              |
  52. +------+--------------+
  53. 1 row in set (0.00 sec)
  54.  
  55.  
  56. # 关键字 in：集合查询
  57. mysql> select name,salary from employee where salary=3000 or salary=3500 or salary=4000 or salary=9000;
  58. +------------+---------+
  59. | name       | salary  |
  60. +------------+---------+
  61. | yuanhao    | 3500.00 |
  62. | jingliyang | 9000.00 |
  63. +------------+---------+
  64. 2 rows in set (0.00 sec)
  65.  
  66. mysql> select name, salary from employee where salary in(3000, 3500, 4000, 9000);
  67. +------------+---------+
  68. | name       | salary  |
  69. +------------+---------+
  70. | yuanhao    | 3500.00 |
  71. | jingliyang | 9000.00 |
  72. +------------+---------+
  73. 2 rows in set (0.00 sec)
  74.  
  75. mysql> select name,salary,post from employee where post='operation' and (salary not in(10000.13));
  76. +-----------+----------+-----------+
  77. | name      | salary   | post      |
  78. +-----------+----------+-----------+
  79. | 程咬金    | 20000.00 | operation |
  80. | 程咬银    | 19000.00 | operation |
  81. | 程咬铜    | 18000.00 | operation |
  82. | 程咬铁    | 17000.00 | operation |
  83. +-----------+----------+-----------+
  84. 4 rows in set (0.00 sec)
  85.  
  86.  
  87. # 关键字 link：模糊查询
  88. # 通配符"%"：任意多字符
  89. mysql> select * from employee where name like '程咬%';
  90. +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
  91. | id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
  92. +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
  93. | 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
  94. | 16 | 程咬银    | female |  18 | 2013-03-11 | operation | NULL         | 19000.00 |    403 |         3 |
  95. | 17 | 程咬铜    | male   |  18 | 2015-04-11 | operation | NULL         | 18000.00 |    403 |         3 |
  96. | 18 | 程咬铁    | female |  18 | 2014-05-12 | operation | NULL         | 17000.00 |    403 |         3 |
  97. +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
  98. 4 rows in set (0.01 sec)
  99.  
  100. # 通配符"_"：任意单字符
  101. mysql> select * from employee where name like 'ale_';
  102. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  103. | id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
  104. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  105. |  2 | alex | male |  78 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |
  106. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  107. 1 row in set (0.00 sec)
  108.  
  109.  
  110. # 其它查询
  111. 1.查看岗位描述不为NULL的员工信息
  112. select * from employee where post_comment is not null;
  113.  
  114. 2. 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
  115. select name,age,salary from employee where post='teacher' and salary not in(10000, 9000, 30000);
  116.  
  117. 3. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
  118. select name,salary*12 from employee where post='teacher' and name like 'jin%';

group by 分组查询

1. 首先声明一点：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：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.

  1. # 需要设置查询模式：only_full_group_by，如果没有设置，查询的结果默认是组内的第一条记录，没有任何意义
  2.  
  3. # 如果想分组，必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
  4. mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
  5. Query OK, 0 rows affected (0.00 sec)
  6.  
  7. # 查看MySQL 5.7默认的sql_mode如下：
  8. mysql> select @@global.sql_mode;
  9. +--------------------+
  10. | @@global.sql_mode  |
  11. +--------------------+
  12. | ONLY_FULL_GROUP_BY |
  13. +--------------------+
  14. 1 row in set (0.00 sec)
  15.  
  16. # 设置成功后，一定要退出，然后重新登录方可生效
  17. mysql> exit;
  18. Bye
  1. # 简单示例：根据部门进行分组查询：
  2. mysql> select post from employee group by post;
  3. +-----------------------------------------+
  4. | post                                    |
  5. +-----------------------------------------+
  6. | operation                               |
  7. | sale                                    |
  8. | teacher                                 |
  9. | 老男孩驻沙河办事处外交大使                  |
  10. +-----------------------------------------+
  11. 4 rows in set (0.00 sec)

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.

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
  • group_concat(字段名)        # 拼接

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.

  1. # 查每个部门有多少个员工
  2. select post,count(1) from employee group by post;    # count(1)："1"的课换成"*"或字段名，好像是新版本上"1"跟"*"的速度基本无差异，
  3.  
  4. # 查每个部门的平均薪水
  5. select post,avg(salary) from employee group by post;
  6.  
  7. # 查岗位名以及岗位包含的所有员工名字
  8. select post,group_concat(name) as name from employee group by post;    # as name：重命名显示的字段名
  9.  
  10. # 查公司内男员工和女员工的个数
  11. select sex,count(id) from employee group by sex;
  12.  
  13. # 查岗位名以及各岗位的最高薪资
  14. select post,max(salary) from employee group by post;
  15.  
  16. # 查男员工与男员工的平均薪资，女员工与女员工的平均薪资
  17. select sex,avg(salary) from employee group by sex;

having 过滤

  1. 执行优先级从高到低：where > group by > having
  2. 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
  3. having发生在分组group by之后，因而having中可以使用分组的字段，无法直接取到其它字段，可使用聚合函数
  1. mysql> select * from employee having salary>100000;    # 可能老版本不能直接使用此语法
  2. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  3. | id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
  4. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  5. |  2 | alex | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
  6. +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
  7. 1 row in set (0.04 sec)
  8.  
  9. # 住：必须使用group by才能使用group_concat()函数
  10. # 错误，分组后无法直接取到组外的字段
  11. mysql> select post,group_concat(name) from employee group by post having salary > 1000000;
  12. ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
  1. # 示例
  2.  
  3. 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
  4. select post,group_concat(name),count(1) from employee group by post having count(id)<2;
  5.  
  6. 2. 查询各岗位平均薪资大于10000的岗位名、平均工资
  7. select post,avg(salary) from employee group by post having avg(salary)>10000;
  8.  
  9. 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
  10. select post,avg(salary) from employee group by post having avg(salary)>10000 and avg(salary)<20000;

order by 查询排序

按单列排序：

  • select * from employee order by age;        # 默认升序，根据age升序
  • select * from employee order by age asc;      # asc 升序
  • select * from employee order by age desc;      # desc 降序

按多列排序：

示例：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;

limit 限制查询的记录数

  1. # 示例
  2.  
  3. 1. 查询前三行记录
  4. select * from employee limit 3;
  5.  
  6. 2. 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
  7. select * from employee order by salary desc limit 0, 5;
  8.  
  9. 3. 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
  10. select * from employee order by salary desc limit 5, 5;
  11.  
  12.  
  13. 4. 从偏移5开始，往后取10条数据
  14. limit 10 offset 5
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原文地址：https://www.cnblogs.com/zyk01/p/11375938.html