mysql - 连接三个 MySQL 表，这样我就不会得到重复数据

Question

我有三个 mysql 表 tblOne、tblTwo 和 tblThree

SELECT tblOne.bookID AS bookID,
  tblOne.title AS title,
  tblOne.author AS author,
  tblOne.blurb AS blurb,
  tblOne.isbn AS isbn,
  tblOne.coverImage As coverImage,
  CONCAT_WS(" ", tblThree.firstName, tblThree.lastName) AS fullName,
  tblTwo.rating,
  tblTwo.reviewText AS review,
  CONCAT_WS(" ", tblTwo.reviewDate, tblTwo.reviewTime) AS reviewDate
FROM tblTwo
INNER JOIN tblOne
  ON tblTwo.bookID = tblOne.bookID
INNER JOIN tblThree
  ON tblTwo.userID = tblThree.userID
WHERE tblTwo.bookID = 1000102;

输出填充一个 XML 文件，对于这本书，有两个评论，因此 title、author 等的数据被拉了两次。我该如何克服这个问题，因为当我尝试从 JavaScript 查询中访问标签时，它失败了，因为它说它是未定义的。我认为这是因为有不止一个结果。

上面的 SQL 实际上确实有效，我的 PHP 是错误的，但是......

如何在没有review的情况下返回图书详情？目前，只有经过审核的图书才会返回其详细信息。

Answer 1

您正在使用 INNER JOIN，这需要行存在于联接表中。请改用 LEFT JOIN，即使左连接表中的行不存在，它也会从第一个列出的表中返回行:

SELECT tblBooks.bookID AS bookID,
    tblBooks.title AS title,
    tblBooks.author AS author,
    tblBooks.blurb AS blurb,
    tblBooks.isbn AS isbn,
    tblBooks.coverImage As coverImage,
    CONCAT_WS(' ', tblMembers.firstName, tblMembers.lastName) AS fullName,
    IFNULL(tblReviews.rating, '0'),
    tblReviews.reviewText AS review,
    CONCAT_WS(' ', tblReviews.reviewDate, tblReviews.reviewTime) AS reviewDate
FROM tblBooks 
LEFT JOIN tblReviews
    ON tblReviews.bookID = tblBooks.bookID
LEFT JOIN tblMembers
    ON tblReviews.userID = tblMembers.userID
GROUP BY 1, 2, 3, 4, 5, 7, 8;

请注意，我在 FROM 子句中列出了 tblOne first，这是获取所有书籍所必需的，即使有些书籍没有评论。

当没有评论时，您将不得不处理 null 值。考虑使用 IFNULL() 生成空白或默认值，例如

IFNULL(tblTwo.rating, '') -- instead of just tblTwo.rating

或者

IFNULL(tblTwo.reviewText, 'None') -- instead of just tblTwo.reviewText

等等