SQL 将 varchar 转换为 int

Question

我试图在我的数据库中显示两个日期（列）之间的天数。输出显示为 varchar 数据类型。我应该如何将其转换为 int 数据类型？以下是弹出给我的错误消息：

消息 245，第 16 级，状态 1，第 42 行

转换 varchar 值“过期天数”时转换失败 数据类型为 int。

我尝试过同时使用 CAST 和 CONVERT 语法，但它们都不起作用。

SELECT DVD_mem_id AS 'Member ID',
       DVD_mem_name AS 'Member Name',
       DVD_movie_id AS 'Movie ID',
       DVD_movie_title AS 'Movie Title',
       DVD_borrowing_date AS 'Date of Borrow',
       DVD_date_due AS 'Date Due',
       CAST('Overdue Day(s)' AS int),
       DATEDIFF(DAY, DVD_borrowing_date, DVD_date_due) -3 AS 'Overdue Day(s)',
       'Overdue Day(s)' * 2 AS 'Fine Incurred'
FROM DVD_T
WHERE (DVD_mem_id IS NOT NULL) AND
      (DVD_mem_name IS NOT NULL) AND
      (DVD_borrowing_date IS NOT NULL) AND
      (DVD_date_due) IS NOT NULL AND
      ('Overdue Day(s)' > 0)

Answer 1

我认为你需要的是这个，正如@jarlh 所说，'Overdue Day(s)' 是一个字符串，你不能在其中进行任何数学运算。

SELECT DVD_mem_id AS 'Member ID',
       DVD_mem_name AS 'Member Name',
       DVD_movie_id AS 'Movie ID',
       DVD_movie_title AS 'Movie Title',
       DVD_borrowing_date AS 'Date of Borrow',
       DVD_date_due AS 'Date Due',
       CAST((DATEDIFF(DAY, DVD_borrowing_date, DVD_date_due) -3) AS int),
       DATEDIFF(DAY, DVD_borrowing_date, DVD_date_due) -3 AS 'Overdue Day(s)',
       (DATEDIFF(DAY, DVD_borrowing_date, DVD_date_due) -3) * 2 AS 'Fine Incurred'
FROM DVD_T
WHERE (DVD_mem_id IS NOT NULL) AND
      (DVD_mem_name IS NOT NULL) AND
      (DVD_borrowing_date IS NOT NULL) AND
      (DVD_date_due) IS NOT NULL AND
      (DATEDIFF(DAY, DVD_borrowing_date, DVD_date_due) -3 > 0)