1. 獲取昨天的日期
   日期相減函數 datediff

select datediff(curdate(),INTERVAL 1 DAY) ;

2. 刪除Person表中重復的Email數據，只保留id最小的一條

delete p1 from Person p1,Person p2 where p1.Email = p2.Email and p1.Id > p2.Id;

3. 換座位，1與2換，3與4換，5保留原來的
   COALESCE空值表達式，第一個不為空，返回第一個結束；否則繼續判斷接下來的數據；

select id, COALESCE(s2.student,s1.student) from seat s1, LEFT JOINseat s2 ON ((s1.id + 1) ^ 1) - 1 = s2.id
ORDER BY s1.id;

4. 交換性別，‘f’ 變 ‘m’, ‘m’ 變 ‘f’

update salary set sex= case sex when 'f' then 'm' else 'f'END;

5. 月份列變行，求總和，并匯總
   if(month=‘Jan’,revenue,null) Jan_Revenue
   case month when ‘Jan’ then revenue END Jan_Revenue

select id,
sum(if(month='Jan',revenue,null)) Jan_Revenue,
sum(if(month='Feb',revenue,null)) Feb_Revenue,
sum(if(month='Mar',revenue,null)) Mar_Revenue,
sum(if(month='Apr',revenue,null)) Apr_Revenue,
sum(if(month='May',revenue,null)) May_Revenue,
sum(if(month='Jun',revenue,null)) Jun_Revenue,
sum(if(month='Jul',revenue,null)) Jul_Revenue,
sum(if(month='Aug',revenue,null)) Aug_Revenue,
sum(if(month='Sep',revenue,null)) Sep_Revenue,
sum(if(month='Oct',revenue,null)) Oct_Revenue,
sum(if(month='Nov',revenue,null)) Nov_Revenue,
sum(if(month='Dec',revenue,null)) Dec_Revenue from Department group by id;select id,
sum(case month when 'Jan' then revenue END) Jan_Revenue,
sum(case month when 'Feb' then revenue END) Feb_Revenue,
sum(case month when 'Mar' then revenue END) Mar_Revenue,
sum(if(month='Apr',revenue,null)) Apr_Revenue,
sum(if(month='May',revenue,null)) May_Revenue,
sum(if(month='Jun',revenue,null)) Jun_Revenue,
sum(if(month='Jul',revenue,null)) Jul_Revenue,
sum(if(month='Aug',revenue,null)) Aug_Revenue,
sum(if(month='Sep',revenue,null)) Sep_Revenue,
sum(if(month='Oct',revenue,null)) Oct_Revenue,
sum(if(month='Nov',revenue,null)) Nov_Revenue,
sum(if(month='Dec',revenue,null)) Dec_Revenue from Department group by id;

總結

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