Question

可能重复：
How to Join three tables

我有4张桌子：

文件夹（id_folder，name）
子文件夹（id_subfolder，id_folder，名称，othercolumn，...）
件（id_piece，id_subfolder，id_folder）
图像（id_image，id_piece，名称，..）

如何获取一个文件夹的所有内容（例如“id_folder = 1”）？

Answer 1

假设这些表之间存在从零到多的关系，这样可以起作用：

SELECT folder.id_folder, folder.name, subfolder.id_subfolder, subfolder.name, subfolder.othercolumn, piece.id_piece, image.id_image, image.name
FROM folder
LEFT JOIN subfolder
ON folder.id_Folder = subfolder.id_folder
LEFT JOIN Piece
ON subfolder.id_subfolder = piece.id_subfolder
LEFT JOIN [image]
on piece.id_piece = image.id_piece
WHERE folder.id_folder = 1;

Answer 2

你会JOIN你的表格，如下所示：

select *
from folder f
left join subfolder sf
    on f.id_folder = sf.id_folder
left join piece p
    on sf.id_subfolder = p.id_subfolder
    and sf.id_folder = p.id_folder
left join image i
    on p.id_piece = i.id_piece
where f.id_folder = 1

如果您需要帮助学习JOIN语法，那么有一个很棒的Visual Explanation of SQL Joins会有所帮助。

如何在2个以上的表Sql Server之间创建一个联合

2 个答案: