musql 单表查询-白红宇

musql 单表查询

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发布时间：2019-06-12

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一介绍

本节内容:

查询语法

关键字的执行优先级

简单查询

单条件查询:WHERE

分组查询:GROUP BY

HAVING

查询排序:ORDER BY

限制查询的记录数:LIMIT

使用聚合函数查询

使用正则表达式查询

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);

二查询语法

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

三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.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,如果有聚合函数也是先执行聚合再having过滤

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

四简单查询

#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DISTINCT    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 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;

五 WHERE约束

强调：where是一种约束条件，mysql会拿着where指定的条件去表中取数据，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 'egon%'

pattern可以是%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;    #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，不是null    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    SELECT name,salary FROM employee         WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;        SELECT name,salary FROM employee         WHERE salary IN (3000,3500,4000,9000) ;    SELECT name,salary FROM employee         WHERE salary NOT IN (3000,3500,4000,9000) ;#6:关键字LIKE模糊查询    通配符’%’    SELECT * FROM employee             WHERE name LIKE 'eg%';    通配符’_’    SELECT * FROM employee             WHERE name LIKE 'al__';

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%';

六分组查询: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，想要获取组内的其他相关信息，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 employee GROUP BY post;#按照岗位分组，并查看组内成员名    SELECT post,GROUP_CONCAT(name) as emp_members 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

#!!!MySQL 5.7默认ONLY_FULL_GROUP_BY语义介绍 #参考链接：http://www.ywnds.com/?p=8184#分组查询的常见问题：mysql> select id,count from tt group by id;ERROR 1055 (42000): Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test.tt.count' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by#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#去掉ONLY_FULL_GROUP_BY模式，如下操作：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';#！！！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。

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5mysql> 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6mysql> 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 |+--------+---------------+

七使用聚合函数查询

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 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;    SELECT SUM(salary) FROM employee WHERE depart_id=3;

八 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;

HAVING与WHERE不一样的地方在于!!!!!!

#！！！执行优先级从高到低：where > group by > 聚合函数 > having #1. Where 是一个约束声明，使用Where约束来自数据库的数据，Where是在结果返回之前起作用的（先找到表，按照where的约束条件，从表(文件)中取出数据），Where中不能使用聚合函数。#2. Having是一个过滤声明，是在查询返回结果集以后对查询结果进行的过滤操作（先找到表，按照where的约束条件，从表(文件)中取出数据，然后group by分组，如果没有group by则所有记录整体为一组，然后执行聚合函数，然后使用having对聚合的结果进行过滤），在Having中可以使用聚合函数。#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(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

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

九查询排序:ORDER BY

按单列排序    SELECT * FROM employee ORDER BY salary;    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;

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1mysql> select * 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 |+-----------+---------------+#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 |+-----------+---------------+

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 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：

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 |+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+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 |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+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 |+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+rows in set (0.00 sec)

十一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/Xanderzyl/p/10957694.html

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BZOJ.3160.万径人踪灭(FFT Manacher)

一 介绍

二 查询语法

三 关键字的执行优先级(重点)

四 简单查询

五 WHERE约束

六 分组查询:GROUP BY

七 使用聚合函数查询

八 HAVING过滤

九 查询排序:ORDER BY

十 限制查询的记录数:LIMIT

十一 使用正则表达式查询

一介绍

二查询语法

三关键字的执行优先级(重点)

四简单查询

六分组查询:GROUP BY

七使用聚合函数查询

九查询排序:ORDER BY

十限制查询的记录数:LIMIT

十一使用正则表达式查询