Question

I need your help building a SQL statement I can’t wrap my head around.

In a database, I have four tables – files, folders, folders_files and links.

I have many files. One of them is called “myFile.txt”.

I have many folders. “myFile.txt” is in some of them. The first folder it appears in is called “firstFolder”.

I have many links to many folders. The first link to “firstFolder” is called “firstLink”.

The data structure for the example would be:

// files
Id: 10
Name: "myFile.txt"

// folders
Id: 20
Name: "firstFolder"

// folder_files (join table)
Id: 30
Folder_Id: 20 (meaning "firstFolder")
File_Id: 1 (meaning "myFile.txt")

// links
Id: 40
Name: "firstLink"
Folder_Id: 20 (meaning "firstFolder")

FIRST QUESTION: How do I get the record for “myFile.txt” AND the Name and Id of “firstLink” (the first link), querying on file Id = 10, based on the lowest Id of the folder and the link?

SECOND QUESTION: How do I get the record for “myFile.txt” AND the Name and Id of “firstLink” (the first link), querying on all files, based on the lowest Id of the folder and the link?

put another way – how do I get the first link to the first folder containing “myFile.txt”?

Resulting in a record that looks like:

Id: 10
Name: "myFile.txt"
LinkId: 40
LinkName: "firstLink"

Thanks!

Answer 1

You should try to think about how you want your result set to look. SQL is designed to describe result sets. If you can write out a hypothetical result set, you might have an easier time writing SQL that will render that result set.

I had a hard time understanding what you are looking for, but I’m sure it’s a fairly straight forward problem. I would be able to help you easier if you could describe you results more clearly, although you might not need my help anymore!

For example (going with you original schema) Q1 & Q2:

files.Id, files.Name, links.Id, links.Name (4 columns)

Q1:

SELECT
  files.Id, files.Name, links.Id, links.Name
FROM
  files, links
INNER JOIN
  folder_files
  ON files.Id = folder_files.File_Id
INNER JOIN
  links
  ON links.Id = folder_files.Folder_Id
WHERE
  files.Id = 10
ORDER BY
  folder_files.File_Id ASC, links.Id ASC
LIMIT 1;

(JOIN with folders table not necessary)

Q2:

Change both ASC to DESC