数据库 单表查询
阅读目录
- 一 单表查询的语法
- 二 关键字的执行优先级(重点)
- 三 where约束
- 四 分组查询 group by
- 五 having 过滤
- 六 查询排序 order by
- 七 限制查询的记录数 limit
- 八 使用正则查询 regexp
一: 单表查询的语法
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
#单表查询语法： select distinct 字段1，字段2，字段3。。。。 from 表名 where 约束条件 group by 分组的字段 having 过滤条件 order by 排序字段 limit 限制条件; #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() 函数用于拼接字符串 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; #使用正则表达式 regexp select * from employee where name regexp '^e.*n$'; #字符串匹配 egon
练习
1 查出所有员工的名字，薪资,格式为
<名字:egon> <薪资:3000>
select concat('<名字:',name,'> ','<薪资:',salary,'>') from employee;
2 查出所有的岗位（去掉重复）
select distinct post from employee;
3 查出所有员工名字，以及他们的年薪,年薪的字段名为annual_year
select name,salary*12 annual_year from employee;
二: 关键字的执行优先级(重点)
重点中的重点：关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit
1.找到表:from
2.拿着where指定的约束条件，去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
4.将分组的结果进行having过滤
5.执行select (要展示的哪些内容)
6.去重
7.将结果按条件排序：order by
8.限制结果的显示条数
详细见：http://www.cnblogs.com/linhaifeng/articles/7372774.html
三: where 约束
#where 约束条件 1. 比较运算符：> < >= <= <> != 2. between 80 and 100 值在80到100之间 3. in(80,90,100) 值是10或20或30 4. like 'egon%' 两种模式可以是%或_， %表示任意多字符 _表示任意一个字符 5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not
#1:单条件查询 SELECT name FROM employee WHERE post='sale'; #2:多条件查询 SELECT name,salary FROM employee WHERE post='teacher' AND salary>10000; #3:关键字BETWEEN AND 在...到...之间 SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000; #4:关键字is null(判断某个字段是否为NULL不能用等号，需要用IS) SELECT name,post_comment FROM employee WHERE post_comment IS NULL; SELECT name,post_comment FROM employee WHERE post_comment IS NOT NULL; #非空在前面加not SELECT name,post_comment FROM employee WHERE post_comment=''; 注意''是空字符串，不是null ps： 执行 update employee set post_comment='' where id=2; 再用上条查看，就会有结果了 #5:关键字IN集合查询 相当于or SELECT name,salary FROM employee WHERE salary=3000 or salary=3500 or salary=4000 or salary=9000 ; SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; #前面加not代表非 #6:关键字LIKE模糊查询 通配符’%’ SELECT * FROM employee WHERE name LIKE 'eg%'; 通配符’_’ SELECT * FROM employee WHERE name LIKE 'al__';
#7:正则表达式 regexp
select * from employee where name regexp '^e.*n$'; #字符串匹配 egon
练习
1. 查看岗位是teacher的员工姓名、年龄
select name,age from employee where post='teacher'; 2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
select name,age from employee where post='teacher' and age > 30; 3. 查看岗位是teacher且薪资在9000-10000范围内的员工姓名、年龄、薪资
select name,age,salary from employee where post='teacher' and salary between 9000 and 10000; 4. 查看岗位描述不为NULL的员工信息
select * from employee where post is not null; 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
select name,age,salary from employee where salary in(10000,9000,30000); 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
select name,age,salary from employee where salary not in(10000,9000,30000);
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
select name,salary*12 from employee where name like 'jin%';
四: 分组查询 group by
1.什么是分组,为什么要分组?注意事项
#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 #5.强调 1、分组之后，select只能查看到分组的字段，要想查组内内容 不能直接查看，需要借助于聚合函数max，min，avg，sum，count 2、分组的目的是为类以组为单位来处理记录，而不是处理单独的记录 3、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
2.设置 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 #！！！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';
查看:
mysql> select @@global.sql_mode; +-------------------+ | @@global.sql_mode | +-------------------+ | | +-------------------+ 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 | +----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+ 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 Bye mysql> 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 | +----------------------------+-----------+ 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 查看组内成员 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; #as改名 GROUP BY与聚合函数count()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
#查询总员工数:没有分组，默认整体一组
select count(id) from employee;
#查看每个部门的员工数
select post,count(id) from employee group by post;
#查看teacher部门的员工数
select count(id) from employee where post='teacher';
#查看每个部门的最高工资
select post,max(salary) from employee group by post;
#查看每个部门的最低工资
select post,min(salary) from employee group by post;
#查看每个部门的平均工资
select post,avg(salary) from employee group by post;
#查看每个部门的工资总和
select post,sum(salary) from employee group by post;
#查看每个部门的名字和人数
select post,count(id) from employee group by post;
#查看每个部门有哪些员工
select post,group_concat(name) from employee group by post;
#查看每个部门员工的薪资,字符串拼接
select post,group_concat('薪资: ',salary) from employee group by post;
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的内容，若是没有分组，则默认一组 示例： SELECT COUNT(*) FROM employee; #计数 SELECT COUNT(*) FROM employee WHERE depart_id=1; SELECT MAX(salary) FROM employee; #最大值 SELECT MIN(salary) FROM employee; #最小值 SELECT AVG(salary) FROM employee; #平均值 SELECT SUM(salary) FROM employee; #求和 SELECT SUM(salary) FROM employee WHERE depart_id=3;
5.练习
1. 查询岗位名以及岗位包含的所有员工名字
select post,group_concat(name) from employee group by post; 2. 查询岗位名以及各岗位内包含的员工个数
select post,count(id) from employee group by post; 3. 查询公司内男员工和女员工的个数
select sex,count(id) from employee group by sex;
4. 查询岗位名以及各岗位的平均薪资
select post,avg(salary) from employee group by post;
5. 查询岗位名以及各岗位的最高薪资
select post,max(salary) from employee group by post;
6. 查询岗位名以及各岗位的最低薪资
select post,min(salary) from employee group by post;
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
select sex,avg(salary) from employee group by sex;
五: 过滤 having
having与where不一样的地方
#！！！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
#3. 易错误：having是在分组之后的，意味着，只能用分组的字段或者聚合函数作为过滤条件
mysql> select @@sql_mode; +--------------------+ | @@sql_mode | +--------------------+ | ONLY_FULL_GROUP_BY | +--------------------+ 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 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ row in set (0.00 sec) mysql> select * from emp having salary > 100000;#只能用分组的字段或聚合函数 ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause 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 'salary' in 'having clause' select * from employee having count(id) > 10000; #错误 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 | +-----------+-------------------------------------------------------+ rows in set (0.00 sec)
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
select post,group_concat(name),count(id) from employee group by post having count(id) < 2;
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary) > 10000;
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
select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) < 20000;
六: order by 字段名 desc(降序,默认升序) 排序
按单列排序 SELECT * FROM employee ORDER BY salary; #按薪资排序,默认升序 SELECT * FROM employee ORDER BY salary ASC;#按薪资升序排序 SELECT * FROM employee ORDER BY salary DESC;#按薪资降序排序 按多列排序:先按照age排序，如果年纪相同，则按照薪资降序排序 SELECT * from employee ORDER BY age, salary DESC;
#先按照age升序排，如果age相同，则id降序排序
select * from employee order by age asc,id desc;
#查看每个部门的最高工资并排序
select post,max(salary) from employee group by post order by max(salary);
select post,max(salary) as m from employee group by post order by m;
#错误，执行having时，select还没有执行呢
select post,avg(salary) as x from employee group by post having x > 10000;
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
select * from employee order by age,hire_date desc; 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
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary); 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
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
七: 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
练习
分页显示,每页五条
select * from employee order by id limit 5;
select * from employee order by id limit 5,5;
select * from employee order by id limit 10,5;
select * from employee order by id limit 15,5;
八: 使用正则表达式查询 regexp
SELECT * FROM employee WHERE name REGEXP '^ale'; SELECT * FROM employee WHERE name REGEXP 'on$'; SELECT * FROM employee WHERE name REGEXP 'm{2}'; 小结：对字符串匹配的方式 WHERE name = 'egon'; WHERE name LIKE 'yua%'; WHERE name REGEXP 'on$';
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.*[ng]$';
既然选择了远方，便是风雨兼程...
