MySQL“加入为”？

Question

我有两个连接在一起的表：

是.book_types_purchased​ ENUM('paperback','hardcover')我想返回这种类型的行：

书名 | 平装本购买日期 | 精装_购买日期

MySQL 有没有办法做到这一点？

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更新

这是下面的答案，其中包含匹配的表名称和左连接：

SELECT title,
  paperback.date_purchased AS paperback_date_purchased,
  hardcover.date_purchased AS hardcover_date_purchased

FROM book_projects

LEFT JOIN book_types_purchased AS paperback 
  ON paperback.book_project_id = book_projects.id
  AND paperback.type = 'paperback'

LEFT JOIN book_types_purchased AS hardcover 
  ON hardcover.book_project_id = book_projects.id
  AND hardcover.type = 'hardcover'

Answer 1

你可以JOIN上桌book_types_purchased两次。每种类型一个。

SELECT title,
  paperback.date_purchased AS paperback_date_purchased,
  hardcover.date_purchased AS hardcover_date_purchased

FROM book_projects

JOIN date_purchased AS paperback ON paperback.book_project_id = book_projects.id
  AND paperback.type = 'paperback'

JOIN date_purchased AS hardcover ON hardcover.book_project_id = book_projects.id
  AND hardcover.type = 'hardcover'