1.单表查询
前面做了大量工作进行表格数据创建，但是数据越多，该怎么使用呢？？？如何从表中关联的表找到自己需要的数据，这个操作很重要
1.1单表查询
语法
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、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 6.去重 7.将结果按条件排序：order by 8.限制结果的显示条数
老师代码演示
company.employee
员工id id int
姓名 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
创建表格的代码
#创建表,设置字段的约束条件 create table employee( id int primary key 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 depart_id int ); #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), ('xiaomage','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) ;
1.2where的使用
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10或20或30 4.like '%': 可以是%或者_。%表示任意多字符，_表示一个字符，like'e_' 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
案例
# 查出id大于5的员工姓名 select name from employee where id>5; # 查出员工身份是老师且薪资大于10000的员工姓名 select name from employee where post='teacher' and salary>10000; # 查出员工薪资介于10000到20000的员工姓名及薪资 select name,salary from employee where salary between 10000 and 20000; # 给id=2的员工职位描述更改为空 update employee set post_comment='' where id=2; # 查出职位描述为空的员工姓名 select name,post_comment from employee where post_comment=''; # 查询薪资是3000或3500或4000或9000的员工姓名以及薪资 select name,salary from employee where salary=3000 or salary=4000 or salary=9000; select name,salary from employee where salary in(3000,4000,9000); # 关键字模糊查询 # 查询以eg开头的员工姓名 select * from employee where name like 'eg__'; # 你大爷居然是两条杠 select * from employee where name like 'eg%';
1.3 常用聚合函数
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() # 查详细信息
1.4 分组（group）查询
分组是基于where之后得到的记录而进行的
group常与聚合和函数结合使用,例：
# 给岗位进行分组 select post from employee group by post; # group by关键字和count()函数结合使用 # 岗位分组，并查看每个岗位人数 select post,count(id) as count from employee group by post; # group by关键字和group_count()函数结合使用 # 岗位分组，并查看每个岗位人员 select post,group_concat(name) from employee group by post; # 给他改个名,改为members select post,group_concat(name) as numbers from employee group by post; # 岗位分组，查看每个岗位人员，并统计人数 select post,group_concat(name) as members,count(id) from employee group by post; # 查询公司内男员工和女员工的个数 select sex,count(id) from employee group by sex; # 查询岗位名以及各岗位最高工资 select post,max(salary) from employee group by post; # 查询男员工与男员工平均薪资，女员工与女员工平均薪资 select sex,avg(salary) from employee group by sex;
1.5 having过滤
having和where语法上一样，但是他两的执行优先级不同：
优先级从高到低：where > group by > having
# 查询薪资大于100000的员工信息（下面两个结果一样） select * from employee where salary>100000; select * from employee having salary>100000; # 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; # 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)>1000; # 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 post,avg(salary) from employee group by post having avg(salary) between 10000 and 20000;
1.6查询排序（order by）
升序 asc
降序desc
单列排序 select * from employee order by salary; # 默认升序 select * from employee order by salary asc; # 升序 select * from employee order by salary desc; # 降序 多列排序 # 先按age默认升序，如果年龄相同，再按薪资升序，如果薪资相同，再按id降序 select * from employee order by age,salary asc,id desc;
1.7 限制查询记录数（limit）
# 查询前三条数据 select * from employee limit 3; # 查询后三条数据 select * from employee order by id desc limit 3; # 从第六条数据开始查询后三条(这种操作常用来分页) select * from employee limit 6,3; # 分页显示，每页5条 select * from employee limit 0,5; select * from employee limit 5,5; select * from employee limit 10,5;
1.8 其它神操作
# 查出所有岗位（去重） select distinct depart_id from employee; # 查出所有员工名字以及他们年薪 select name,salary*12 年薪 from employee;