MySQL：列出其他表中的數據及其相關數據（如果有）

Question

這是我的情況：

表image_book_rel包含兩個表之間的關系，即book_table和image_table 。

我需要從表book_table獲取書籍列表，並從表image_table 獲取相關圖像數據（ 如果有） 。

這些表格大致如下所示（簡化）：

/* 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

這是我查詢結果所需要的：

/* 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

這是我嘗試過的：

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

上面查詢的結果僅檢索具有圖像的書。 我需要檢索所有書籍。 我該怎么辦？

Answer 1

不帶WHERE情況下嘗試使用IFNULL將空值替換為空字符串：

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

結果：

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中查看結果。

Answer 2

您正在混合兩種JOIN語法。 您擁有的WHERE子句正在復制LEFT JOIN子句（這是您想要的），但是它等效於實現INNER JOIN，它排除了沒有圖像的行。

正確的查詢要簡單得多：

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

為了進一步完善答案... WHERE條件

bk.id = bk.id

是絕對沒用的，而WHERE條件：

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

通過濾除具有NULL im.id的行，使您指定的LEFT JOINS無效。 這些條件強制執行與查詢相同的結果：

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