MySQL之单表查询
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 单表查询关键字执行的优先级：
重点中的重点：关键字的执行优先级
from：from后面跟的是表，也就是说我们从哪个表中查询。
where：where后面加的是一些限制条件，比如大于多少，范围多少等。
group by:group by 后面跟的分组条件，也就是按照什么来分组。
having：having是在语法级别上跟where是一样的，唯一的区别是having实在分组之后进行限制，可以使用聚合函数，而where不行
select：select后面跟所要查询的字段信息。
distinct：distinct是用来去重的。
order by：限制显示的顺序，可以按照id，age等显示，最后还可以跟降序（desc）和升序（asc）。
limit：限制一次显示记录的数量，limit 5，name一次显示五条，同样可以limit 1,5表示从1开始显示5条。其中不包括1.
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。
下面我们通过一个实例来演示这些关键字的功能：
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 准备表和记录
1 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; 结合CASE语句： SELECT ( CASE WHEN NAME = 'egon' THEN NAME WHEN NAME = 'alex' THEN CONCAT(name,'_BIGSB') ELSE concat(NAME, 'SB') END ) as new_name FROM emp;
2 where：
where字句中可以使用：
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 '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#between的用法就是后面加上两个数，两个数之间加一个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__';
三 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，这是对于一般情况下而言的。 #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
那么分组的时候有没有指定：set global sql_mode='ONLY_FULL_GROUP_BY'; 是有区别的，没有设置的时候默认查到的是每一个组第一条信息。同样的分完组之后我们不能再直接通过select * from emp来查询组内的信息，
假如我们按照post来分组，那么我们查询的时候只能select post 聚合函数（max（），min（），avg（））from emp group by post等。
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 | +----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+ 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 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 | +----------------------------+-----------+ 4 rows in set (0.00 sec)
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
四 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; 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;
五 HAVING过滤：
#！！！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
六 order by排序：
按单列排序 SELECT * FROM employee ORDER BY salary; SELECT * FROM employee ORDER BY salary ASC; ASC是升序排序，而desc为降序排序。默认是按照升序排序的。 SELECT * FROM employee ORDER BY salary DESC; 分别为ascend和descend 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;
七 limit限制查询的显示记录数：
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
八 使用正则表达式查询：
SELECT * FROM employee WHERE name REGEXP '^ale'; SELECT * FROM employee WHERE name REGEXP 'on$'; SELECT * FROM employee WHERE name REGEXP 'm{2}'; 小结：对字符串匹配的方式 WHERE name = 'egon'; WHERE name LIKE 'yua%'; WHERE name REGEXP 'on$';
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]$';
总结：
一 语法 select distinct 查询字段1，查询字段2，。。。 from 表名 where 分组之前的过滤条件 group by 分组依据 having 分组之后的过滤条件 order by 排序字段 limit 显示的条数; def from(dir,file): open('%s\%s' %(dir,file),'r') return f def where(f,pattern): for line in f: if pattern： yield line def group(): pass def having(): pass def distinct(): pass def order(): pass def limit(): pass def select(): res1=from() res2=where(res1,pattern) res3=group(res2,) res4=having(res3) res5=distinct(res4) res6=order(res5) limit(res6) 二 where过滤 select id,name from db39.emp where id >= 3 and id <= 6 select * from db39.emp where id between 3 and 6; select * from emp where salary = 20000 or salary = 18000 or salary = 17000; select * from emp where salary in (20000,18000,17000); 要求：查询员工姓名中包含i字母的员工姓名与其薪资 select name,salary from db39.emp where name like '%i%' 要求：查询员工姓名是由四个字符组成的的员工姓名与其薪资 select name,salary from db39.emp where name like '____'; select name,salary from db39.emp where char_length(name) = 4; select * from db39.emp where id not between 3 and 6; select * from emp where salary not in (20000,18000,17000); 要求：查询岗位描述为空的员工名与岗位名 select name,post from db39.emp where post_comment is NULL; select name,post from db39.emp where post_comment is not NULL; 三 group by分组 #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 mysql> set global sql_mode="strict_trans_tables,only_full_group_by"; #每个部门的最高工资 select post,max(salary) from emp group by post; select post,min(salary) from emp group by post; select post,avg(salary) from emp group by post; select post,sum(salary) from emp group by post; select post,count(id) from emp group by post; #group_concat（分组之后用） select post,group_concat(name) from emp group by post; select post,group_concat(name,"_SB") from emp group by post; select post,group_concat(name,": ",salary) from emp group by post; select post,group_concat(salary) from emp group by post; # 补充concat（不分组时用） select name as 姓名,salary as 薪资 from emp; select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪资 from emp; # 补充as语法 mysql> select emp.id,emp.name from emp as t1; # 报错，这里报错是因为你已经把表名emp改成t1了，所以再使用emp。id
这个时候没办法找到emp
mysql> select t1.id,t1.name from emp as t1; # 查询四则运算 select name,salary*12 as annual_salary from emp; 分组练习 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 emp 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 emp group by post; 3. 查询公司内男员工和女员工的个数 select sex,count(id) from emp group by sex; 4. 查询岗位名以及各岗位的平均薪资 select post,avg(salary) from emp group by post; 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 select sex,avg(salary) from emp group by sex; 8、统计各部门年龄在30岁以上的员工平均工资 select post,avg(salary) from emp where age >= 30 group by post; 四 having过滤 having的语法格式与where一模一样，只不过having是在分组之后进行的进一步过滤 即where不能用聚合函数，而having是可以用聚合函数，这也是他们俩最大的区别 1、统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门 select post,avg(salary) from emp where age >= 30 group by post having avg(salary) > 10000; #强调：having必须在group by后面使用 select * from emp having avg(salary) > 10000; 五 distinct去重 select distinct post,avg(salary) from emp where age >= 30 group by post having avg(salary) > 10000; 六 order by 排序 select * from emp order by salary asc; #默认升序排ascend select * from emp order by salary desc; #降序排descend select * from emp order by age desc; #降序排 select * from emp order by age desc,salary asc; #先按照age降序排，再按照薪资升序排 # 统计各部门年龄在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排序 select post,avg(salary) from emp where age > 10 group by post having avg(salary) > 1000 order by avg(salary) ; 七 limit 限制显示条数 select * from emp limit 3; select * from emp order by salary desc limit 1; # 分页显示 select * from emp limit 0,5; select * from emp limit 5,5; 八 正则表达式 select * from emp where name regexp '^jin.*(n|g)$';
