单表查询——详解

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;
    上面是语法定义顺序，不是执行顺序；



#使用函数的方式模拟语法顺序；
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


# 语法执行顺序（如果直接写后面的 前面关键字默认值=True）
def select():
    res1=from()
    res2=where(res1,pattern)
    res3=group(res2,)
    res4=having(res3)
    res5=distinct(res4)
    res6=order(res5)
    limit(res6)   #limit 本质是一个打印操作

？？？
select math as "级别",count(math)as "人数" from stu group by "级别" order by "人数" desc limit 0,3;
？？？？

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;

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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;   最大值
    select post,min(salary) from emp group by post;   最小值
    select post,avg(salary) from emp group by post;   平均值
    select post,sum(salary) from emp group by post;    总和
    select post,count(id) from emp group by post;     每个分组的人数



    #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
    mysql> select t1.id,t1.name from emp as t1;



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    # 查询四则运算
    select name,salary*12 as annual_salary from emp;
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    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;

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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;  #这是错误的


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五 distinct去重

select distinct post,avg(salary) from emp
            where age >= 30
            group by post
            having avg(salary) > 10000;


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六 order by 排序
select * from emp order by salary asc; #默认升序排
select * from emp order by salary desc; #降序排

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)
    ;

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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;


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八 正则表达式
# MYSQL支持正则匹配

select * from emp where name regexp '^jin.*(n|g)$';


# 开启正则
select * from emp where name regexp