一 語法

   select distinct 查詢欄位1，查詢欄位2，。。。 from 表名
        where 分組之前的過濾條件
        group by 分組依據
        having 分組之後的過濾條件
        order by 排序欄位
        limit 顯示的條數;
    上面是語法定義順序，不是執行順序；



#使用函式的方式模擬語法順序；
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


# 語法執行順序（如果直接寫後面的 前面關鍵字預設值=True）
def select():
    res1=from()
    res2=where(res1,pattern)
    res3=group(res2,)
    res4=having(res3)
    res5=distinct(res4)
    res6=order(res5)
    limit(res6)   #limit 本質是一個列印操作

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

二 where過濾

    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：查詢員工姓名中包含i字母的員工姓名與其薪資
    select name,salary from db39.emp where name like '%i%'

    要求：查詢員工姓名是由四個字元組成的的員工姓名與其薪資
    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：查詢崗位描述為空的員工名與崗位名
    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分組
    #設定sql_mode為only_full_group_by，意味著以後但凡分組，只能取到分組的依據
    mysql> set global sql_mode="strict_trans_tables,only_full_group_by";

    #每個部門的最高工資
    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;    總和
    select post,count(id) from emp group by post;     每個分組的人數



    #group_concat（分組之後用）
    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;

    # 補充concat（不分組時用）
    select name as 姓名,salary as 薪資 from emp;

    select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪資 from emp;

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



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    # 查詢四則運算
    select name,salary*12 as annual_salary from emp;
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    分組練習

    1. 查詢崗位名以及崗位包含的所有員工名字
        select post,group_concat(name) from emp group by post;


    2. 查詢崗位名以及各崗位內包含的員工個數
        select post,count(id) from emp group by post;

    3. 查詢公司內男員工和女員工的個數
        select sex,count(id) from emp group by sex;

    4. 查詢崗位名以及各崗位的平均薪資
        select post,avg(salary) from emp group by post;
    5. 查詢崗位名以及各崗位的最高薪資
    6. 查詢崗位名以及各崗位的最低薪資
    7. 查詢男員工與男員工的平均薪資，女員工與女員工的平均薪資
        select sex,avg(salary) from emp group by sex;

    8、統計各部門年齡在30歲以上的員工平均工資
       select post,avg(salary) from emp where age >= 30 group by post;

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四 having過濾
    having的語法格式與where一模一樣，只不過having是在分組之後進行的進一步過濾
    即where不能用聚合函式，而having是可以用聚合函式，這也是他們倆最大的區別

    1、統計各部門年齡在30歲以上的員工平均工資，並且保留平均工資大於10000的部門
    select post,avg(salary) from emp
            where age >= 30
            group by post
            having avg(salary) > 10000;

    #強調：having必須在group by後面使用
    select * from emp
            having avg(salary) > 10000;  #這是錯誤的


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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; #預設升序排
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，再按照薪資升序排



# 統計各部門年齡在10歲以上的員工平均工資，並且保留平均工資大於1000的部門，
然後對平均工資進行排序

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 限制顯示條數
select * from emp limit 3;

select * from emp order by salary desc limit 1;



# 分頁顯示
select * from emp limit 0,5;
select * from emp limit 5,5;


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八 正則表示式
# MYSQL支援正則匹配

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


# 開啟正則
select * from emp where name regexp