Question

我在书籍和作者之间有很多联系。

如何获得作者不止一个的所有书籍？（在下面的示例数据库中，该值为0）

books表：

+--------+------------------+
| BookId |       Book       |
+--------+------------------+
|      1 | Wizard of Oz     |
|      2 | Huckleberry Finn |
+--------+------------------+

作者表：

+----------+------------------+
| AuthorId |      Author      |
+----------+------------------+
|        1 | Mark Twain       |
|        2 | Lyman Frank Baum |
+----------+------------------+

连接表：

+--------+----------+
| BookId | AuthorId |
+--------+----------+
|      1 |        2 |
|      2 |        1 |
+--------+----------+

Answer 1

您可以使用分组计数

select books.* from  books
inner join  junction  on books.id  = junction.bookid 
group by bookId 
having count(distinct AuthorId ) > 1

Answer 2

使用distinct count（）

  select * from books where BookId
in (  select BookId from junction_table
    group by BookId
    having count(distinct AuthorId)>=2
   )

Answer 3

此处可以使用CTE（公用表表达式）（请参阅https://docs.microsoft.com/en-us/sql/t-sql/queries/with-common-table-expression-transact-sql）。

首先为CTE命名，指定内部查询，即获取拥有多个作者的所有图书名称，然后使用外部查询提取CTE返回的图书的详细信息。优点是CTE具有模块化和可读性。

WITH CTE 
AS
(
  SELECT Bookid,COUNT(DISTINCT AuthorID) AS No_Of_Authors
  FROM
  Junction
  HAVING COUNT(DISTINCT AuthorID)>1
)
SELECT * FROM CTE  C JOIN Book B
ON
C.Bookid=B.Bookid

Answer 4

请尝试以下操作是否达到您的期望：

SELECT *
FROM
(
    SELECT b, COUNT(b) as authors
    FROM j
    GROUP BY 1
) cnt
WHERE cnt.authors > 1

SQL选择超过1个结点

4 个答案: