如何使用datediff功能加入

Question

create procedure fine() 
begin 
declare a integer(10); 
declare b integer(10); 
declare c integer(10); 
declare d integer(10); 
declare cal_fine cursor for select book_id,datediff(return_date,issue_date) 
from return_book where datediff(return_date,issue_date)>10; 
open cal_fine; 
myloop:loop 
fetch cal_fine into a,b; 
set c=(b-10)*5; 
update return_book set fine=c where book_id=a; 
end loop; 
close cal_fine; 
end; 

我有這段代碼找到很好，但問題是，我的issue_date不在return_book表中，所以我必須加入issue_book和return_book但是那麼如何使用datediff功能？ PLZ幫我

Answer 1

鑑於 DROP TABLE issue_book，return_book;

create table issue_book (book_id int, issue_date date); 
    create table return_book (book_id int, return_date date); 

    insert into issue_book values(1,'2017-01-01'), (2,'2017-09-02'),(3,'2017-09-01'); 
    insert into return_book values(1,'2017-10-08'), (2,'2017-10-08'); 

此查詢

select ib.issue_date,rb.return_date ,datediff(return_date,issue_date) ddiff 
from return_book rb 
join issue_book ib on rb.book_id=ib.book_id 
where datediff(return_date,issue_date)>10; 

結果

+------------+-------------+-------+ 
| issue_date | return_date | ddiff | 
+------------+-------------+-------+ 
| 2017-01-01 | 2017-10-08 | 280 | 
| 2017-09-02 | 2017-10-08 | 36 | 
+------------+-------------+-------+ 
2 rows in set (0.00 sec) 

但也有可能是你的邏輯缺陷，即會發生什麼，如果一本書沒有得到回報 - 將有沒有進入在return_books中。

也不需要一個過程或光標這可以用類似

update return_book rb join issue_book ib on rb.book_id=ib.book_id 
set fine = (datediff(return_date,issue_date) - 10) * 5 
where datediff(return_date,issue_date)>10;