七、单表查询
先创建表
#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','teacher',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) ;
1 1.注意： 2 select * from t1 where 条件 group by 分组字段 3 1.分组只能查询分组字段，要想查看其余的利用聚合函数 4 2.聚合函数的分类：count,min,max,avg,group_concat,sum等。 5 3.模糊匹配：用like关键字。 6 select * from t1 where name like '%eg%'; #%表示任意字符 7 select * from t1 where name like 'd__l'; #一个下划线表示一个字符，两个下划线就表示两个字符 8 4.拷贝表 ：create table t2 select * from t1; 9 create table t2 select * from t1 where 1=2 ;
一、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
二、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> select concat('<名字:',name,'> ' ,'<薪资:',salary,'>' ) from employee; 2 查出所有的岗位（去掉重复） select distinct depart_id from employee; 3 查出所有员工名字，以及他们的年薪,年薪的字段名为年薪 select name,salary*12 年薪 from employee;
三、where约束
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80或90或100
4. like 'eg%'
可以是%或_，
%表示任意多字符
_表示一个字符
like 'e__n' :
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__';
四、having过滤
having和where语法上是一样的。
select * from employee where id>15; select * from employee having id>15;
但是having和where不一样的地方在于以下几点！！！
#！！！执行优先级从高到低：where > group by > 聚合函数 > having >order by
1.where和having的区别 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.where的优先级比having的优先级高 4.having可以放到group by之后，而where只能放到group by 之前。
验证不同之处：
1.查看员工的id>15的有多少个 select count(id) from employee where id>15;#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 id>15; #报错，分析：先执行聚合count(id)，后执行having过滤， #无法对id进行id>15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id>15的过滤，很明显，根本无法获取到id字段
1 ------having----------- 2 select depart_id,count(id) from employee group by depart_id; 3 select depart_id,count(id) from employee group by depart_id having depart_id = 3; 4 select depart_id,count(id) from employee group by depart_id having count(id)>7; 5 select max(salary) 最大工资 from employee where id>2 group by depart_id having count(id)>3; 6 select * from employee where id>7; #查看所有id>7的员工信息
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 select post,group_concat(name) 员工姓名,count(id) 个数 from employee group by post having count(id)<2; 2. 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; 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 select post,avg(salary) from employee group by post having avg(salary) between 10000 and 20000;
五、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：
分组：一般相同的多的话就可以分成一组（一定是有重复的字段）
小练习：
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,max(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;
六、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.如果有聚合函数，则将组进行聚合
5.将4的结果过滤：having
6.查出结果：select
7.去重
8.将6的结果按条件排序：order by
9.将7的结果限制显示条数
七、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;
===========order by========== 1.select * from employee order by salary;#如果不指定，默认就是升序 2.select * from employee order by salary asc; 3.select * from employee order by salary desc; #先按照年龄升序，当年龄相同的太多，分不清大小时，在按照工资降序 4.select * from employee order by age asc, salary desc;
小例子：
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 * form 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; 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 desc;
八、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) form employee WHERE depart_id=3;
九、where的补充（使用正则表达式查询）
1.select * from employee where name regexp '^ale'; #匹配以ale开头的员工信息 2.select * from employee where name regexp 'on$'; #匹配以on结尾的员工信息 3.select * from employee where name regexp 'n{1,2}'; #匹配name里面包含1到2个n的员工信息 小结：对字符串匹配的方式 where name = 'egon'; where name like 'yua%'; where name regexp 'on$';
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.*[ng]$';
十、限制查询的记录数：LIMIT
=========limit：限制打印几条========= 1.select * from employee limit 3;#打印前三条 2.像这样表示的：指的是从哪开始，往后取几条 (这样的操作一般用来分页) select * from employee limit 0,3; select * from employee limit 3,4; select * from employee limit 6,3; select * from employee limit 9,3; 3.select * from employee order by id desc limit 3; #查看后三条
