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 中获取书籍列表，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
下面是我需要的查询结果:
/* 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 有图像的书籍。我需要检索所有 书籍。我该怎么做？
最佳答案
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.
关于MySQL:列出数据及其来自其他表的相关数据(如果可用)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23625912/