Question

给出一个如下表：

| Book in Shelves | Arrived Books | Borrowed Books | Total Books |
        15               10                5              ?

如何添加前三列并将总数显示在Total Books列中？

Book in Shelves + Arrived books - Borrowed Books = Total Books

Answer 1

你的意思是更新＆＃39;总书籍＆＃39;根据你在下面提供的计算？试试这个：

UPDATE `tablename` SET total_books=(books_in_shelves+arrived_books-borrowed_books)

您也可以使用triggers。

＆＃39; PHP和MySQL for dummies＆＃39;在我看来是一本好书。但是Lynda.com的一些网络教程也很有帮助！

Answer 2

在设计数据库时，不存储可以从其他值计算的值。这将使您的数据库不一致。例如：如果您将Book in Shelves更新为20，则必须添加其他查询才能编辑Total Books。所以不要添加一个可以计算的列。从数据库中检索数据时，您可以执行以下操作：

SELECT `Book in Shelves`, `Arrived Books`, `Borrowed Books`, (`Book in Shelves` + `Arrived Books` - `Borrowed Books`) AS `Total books` FROM `table`;

结果：

+-----------------+---------------+----------------+-------------+
| Book in Shelves | Arrived Books | Borrowed Books | Total books |
+-----------------+---------------+----------------+-------------+
|              15 |            10 |              5 |          20 |
|              44 |            22 |              5 |          61 |
|              12 |             2 |              7 |           7 |
+-----------------+---------------+----------------+-------------+

我在MySQL上的第一个ADD / Subtract

2 个答案: