尝试从日期加入2个表和SUM（）时间

Question

我正在尝试编写一个SQL查询，该查询将创建过去一个月内已检出的所有图书的列表，以及每本图书已签出多长时间。

我有books表...

library6=# SELECT * FROM books;
 isbn |                 title                 |    author     
------+---------------------------------------+---------------
  111 | Harry Potter and the Sorcerer’s Stone | JK Rowlings
  222 | The Art of the Deal                   | Trump
  333 | to catch a crook                      | Robert Muller
  444 | Im a cool guy                         | Forrest Stone
  555 | rich i am                             | Bill Gates
(5 rows)

和transactions表，其中包含检出书籍的记录

library6=# SELECT * FROM transactions;
 id | checked_out_date | checked_in_date | user_id | isbn 
----+------------------+-----------------+---------+------
  1 | 2016-01-01       | 2016-02-01      |       1 |  111
  2 | 2016-02-02       | 2016-03-02      |       2 |  111
  3 | 2016-03-03       | 2016-05-03      |       3 |  111
  4 | 2017-12-06       |                 |       4 |  111
  5 | 2017-11-29       | 2017-12-04      |       5 |  555
  6 | 2017-11-28       | 2017-12-05      |       1 |  333
(6 rows)

我的一些更接近的失误是

library6=# SELECT DISTINCT  books.title, books.author  FROM books LEFT OUTER  JOIN transactions ON books.isbn = transactions.isbn ;
                 title                 |    author     
---------------------------------------+---------------
 Harry Potter and the Sorcerer’s Stone | JK Rowlings
 rich i am                             | Bill Gates
 The Art of the Deal                   | Trump
 Im a cool guy                         | Forrest Stone
 to catch a crook                      | Robert Muller
(5 rows)

Answer 1

在mysql中你可以使用datediff（），sum（）和group by

SELECT   b.title
      , b.author 
      , sum(DATEDIFF(t.checked_in_date , t.checked_out_date)) num_days
FROM books b  
LEFT  JOIN transactions t ON b.isbn = t.isbn 
group by  b.title, b.author 

Answer 2

对于postgre：

select b.isbn, 
   b.title, 
   b.author,
   sum(DATE_PART('day', coalesce(t.checked_in_date, CURRENT_DATE), greatest(t.checked_out_date, CURRENT_DATE+ interval '-1 month'))) days_checked_out
  from books b
  join transactions t on b.isbn = t.isbn
 where t.checked_out_date between CURRENT_DATE+ interval '-1 month' and CURRENT_DATE
    or t.checked_in_date  between CURRENT_DATE+ interval '-1 month' and CURRENT_DATE
 group by b.isbn, 
   b.title, 
   b.author;

如果您还需要显示过去一个月未签出的图书，可以为SUM添加左连接和合并。