Question

I need to write a select statement which gathers not only information from multiple tables, but information from within a certain table which matches data in other rows within that same table.

So while joins would work for the multiple table issue, as far as I am aware I cannot “join” column a and column b in the same table.

Here’s the detail

I have the following tables

CR, VM and Folder

I need to select the following:

ID and Name from CR

Folder Name

Folder name however is not in the folder table, to find it, I need to first take “ID” from CR and match it to “ObjectID” in VM – then the column “Location” in that row matches the ObjectID of another row in the VM table.

I need to then match ID in folder to that ObjectID and retrieve “Name” in the table “Folder” to retrieve the folder name”.

I don’t quite know where to start, should I use cursors for this?

Answer 1

It is possible to join a table against itself by aliasing one side of the join. This may be enough to get you started. Otherwise we would need to see some sample data.

SELECT
  CR.ID,
  CR.Name,
  Folder.Name AS FolderName
FROM
  CR
  /* First join CR to VM */
  INNER JOIN VM ON CR.ID = VM.ObjectID
  /* Then join VM to itself aliased as VMLoc, mapping Location to ObjectID */
  INNER JOIN VM AS VMLoc ON VM.Location = VMLoc.ObjectID
  /* Finally join the second ObjectID to Folder to get Folder.Name */
  INNER JOIN Folder ON Folder.ID = VMLoc.ObjectID