MySQL错误：＃1242 - 子查询返回超过1行

Question

SELECT BookId, Duedate, (SELECT Title FROM Book_Information WHERE BookId = BookId)
FROM Transaction_Information WHERE DueDate <= CURDATE() AND ReturnedDate IS NULL

book id是transaction_information中的外键。 我想将书籍信息表中的标题字段添加到交易信息表

Answer 1

由于contains(_:)是外键，它引用了Transaction_Information.BookId表中的相应行，因此一个简单的Book_Information就可以完成这项工作。

以下是查询：

INNER JOIN

Answer 2

尝试此查询：

select * from Transaction_Information tf,Book_Information bf where tf.BookId = bf.BookID and tf.DueDate <= CURDATE() and tf.ReturneDate is null ;