"""
使用情況：等值轉換，範圍轉換，列轉行
操作方式：兩種
1:將case後的欄位值與when後面的值進行比較，簡單分類

    select 欄位1, 欄位2,       
        case 欄位3     
        when 值1 then 新值       
        when 值2 then 新值      
        end as 重新命名欄位3的名字       
    from table      
    where ……      
    order by …… 

2：case後無值，根據when邏輯表示式判定
    
    select 欄位1, 欄位2,       
 

        case 
        when 欄位 > 值1 then 新值       
        when 欄位 < 值1 then 新值  
        else 新值    
        end  as 重新命名欄位3的名字       
    from table      
    where ……      
    order by …… 

"""

原始資料展示

select * from one_buyer

範圍轉換

select id,name,case when the_times >=70 then 'young' else 'old' end as age from
 
 one_buyer;

等值轉換

select id,name,case ser_id_id when 1 then 'fenda' when 2 then 'baisuishan' else 'kele' end as goods from one_buyer

範圍轉換 --- 兩個語句同樣的效果

update one_buyer set gender = case gender > 70 when true then 1 else 0 end;
update  one_buyer set gender = case when gender > 70  then 1 else
 
 0 end;

行轉列

"""
5,6,7為列轉行，進階語句，最終執行7將生成列轉行檢視，6則將查詢結果返回，6為5的分組進階
參考連結：https://blog.csdn.net/weixin_44770933/article/details/106992881?utm_medium=distribute.pc_relevant.none-task-blog-baidujs_title-0&spm=1001.2101.3001.4242
"""

原始資料

sql5 = "select name, case when grade = 'primary' then rate else 0.00 end as 'primary' , case when grade = 'middle' then rate else 0.00 end as 'middle' ,  case when grade = 'high' then rate else 0.00 end as 'high' from mst_class;"

增加分組，並取最大值

sql6 = "select name, max(case when grade = 'primary' then rate else 0.00 end) as 'primary' , max(case when grade = 'middle' then rate else 0.00 end) as 'middle' ,  max(case when grade = 'high' then rate else 0.00 end) as 'high' from mst_class group by name"

建立檢視容器

sql7 = "create view class_grade as select name, max(case when grade = 'primary' then rate else 0.00 end) as 'primary' , max(case when grade = 'middle' then rate else 0.00 end) as 'middle' ,  max(case when grade = 'high' then rate else 0.00 end) as 'high' from mst_class group by name"

補充：

"""
1. IF(expr1,expr2,expr3)：如果第一個條件為True,則返回第二個引數，否則返回第三個
2. ifnull(expr1,expr2) 判斷是否為空：假如第一個引數為null，則返回第二個引數；否則直接返回第一個引數的值
此處不上圖了
"""

sql8 = "select id,name,if(gender=1,'man','women') as sex from one_buyer"
sql9 = "select id,name,case gender =1 when true then 'man' else 'women' end as sex from one_buyer"

sql10 = "select id,name,ifnull(the_user,'ttoia') as the_user from one_ser where id=32"
sql11 = "select id,name,case when the_user is null then 'ttoia' else the_user end as user from one_ser where id =32"

sql12 = "insert into one_ser(name,price,num,is_sail) values('qiaolezi',456,89,1);"
sql13 = "select id,the_user from one_ser where id>30"

待完善。。。