MySQL：列出数据及其来自其他表的相关数据（如果可用）

Question

这是我的场景：

表image_book_rel包含两个表之间的关系，即book_table和image_table。

我需要从表book_table 中获取图书列表，并从表image_table 中获取相关图像数据如果可用。

表格大致如下（简化）：

/* book_table */
id       title      price
-------- ---------- ------
1        Book 1     $10
2        Book 2     $13
3        Book 3     $15
4        Book 4     $20
5        Book 5     $12
6        Book 6     $10
7        Book 7     $14

/* image_table */
id       description      file
-------- ---------------- -------------
20       Image of Book 2  book_img2.jpg
30       Image of Book 3  book_img3.jpg
50                        book_img5.jpg
70       Image of Book 7  book_img7.jpg

/* image_book_rel */
id       book_id   image_id
-------- --------- ---------
1        2         20
2        3         30
3        5         50
4        7         70

这是我的查询结果所需的：

/* Expected result: */
book_id  image_id  filename        book_title  img_desc
-------- --------- --------------- ----------- -----------------
1                                  Book 1
2        20        book_img2.jpg   Book 2       Image of Book 2
3        30        book_img3.jpg   Book 3       Image of Book 3
4                                  Book 4
5        50        book_img5.jpg   Book 5
6                                  Book 6
7        70        book_img7.jpg   Book 7       Image of Book 7

这是我尝试过的：

SELECT bk.id AS book_id, im.id AS image_id, im.file AS filename, bk.title AS book_title, im.description AS img_desc
    FROM book_table AS bk
        LEFT JOIN image_book_rel AS bi
            ON bk.id = bi.book_id
        LEFT JOIN image_table AS im
            ON bi.image_id = im.id
    WHERE bk.id = bk.id
        AND bi.book_id = bk.id
        AND bi.image_id = im.id
    ORDER BY bk.id

上述查询的结果仅检索有图片的图书。我需要检索所有本书。我该怎么办？

Answer 1

尝试不使用WHERE 并使用IFNULL 将空值替换为空字符串：

SELECT IFNULL(bk.id,'') AS book_id, IFNULL(im.id,'') AS image_id, IFNULL(im.file,'') AS filename, IFNULL(bk.title,'') AS book_title, IFNULL(im.description,'') AS img_desc
    FROM book_table AS bk
        LEFT JOIN image_book_rel AS bi
            ON bk.id = bi.book_id
        LEFT JOIN image_table AS im
            ON bi.image_id = im.id
    ORDER BY bk.id

结果：

BOOK_ID   IMAGE_ID  FILENAME        BOOK_TITLE   IMG_DESC
1                                   Book 1       
2         20        book_img2.jpg   Book 2       Image of Book 2
3         30        book_img3.jpg   Book 3       Image of Book 3
4                                   Book 4       
5         50        book_img5.jpg   Book 5       
6                                   Book 6       
7         70        book_img7.jpg   Book 7       Image of Book 7

在SQL Fiddle 中查看结果。

Answer 2

您正在混合使用两种类型的 JOIN 语法。您拥有的 WHERE 子句复制了 LEFT JOIN 子句（这是您想要的），但它相当于实现了一个 INNER JOIN，它排除了没有图像的行。

正确的查询要简单得多：

SELECT bk.id AS book_id, im.id AS image_id, im.file AS filename, bk.title AS book_title, im.description AS img_desc
    FROM book_table AS bk
        LEFT JOIN image_book_rel AS bi
            ON bk.id = bi.book_id
        LEFT JOIN image_table AS im
            ON bi.image_id = im.id
    ORDER BY bk.id

为了进一步完善答案... WHERE 条件

bk.id = bk.id

绝对没用，而 WHERE 条件：

bi.book_id = bk.id
bi.image_id = im.id

通过过滤掉具有 NULL im.id 的行，使您指定的 LEFT JOINS 无效。这些条件会产生与查询相同的结果：

SELECT bk.id AS book_id, im.id AS image_id, im.file AS filename, bk.title AS book_title, im.description AS img_desc
    FROM book_table AS bk
        JOIN image_book_rel AS bi
            ON bk.id = bi.book_id
        JOIN image_table AS im
            ON bi.image_id = im.id
    ORDER BY bk.id