自我加入的替代方案

Question

I have a table that has a reference to itself like this:

Id   Total   Prev   Session
1  | 10    | NULL | 1
2  | 15    | 1    | 1
3  | 11    | NULL | 2
4  | 29    | 2    | 1
5  | 19    | 3    | 2
6  | 47    | 4    | 1

And I need to get the differences for the specific sessions.
Like this for session 1:

1. 10 -- None to 10
2. 5  -- 10 to 15
3. 14 -- 15 to 29
4. 18 -- 29 to 47

To do this, I use:

SELECT  F.Total - P.Total AS Difference
FROM    Foo F LEFT OUTER JOIN
        Foo P ON F.Prev = P.Id
WHERE   Session = @Session

Which is extremely slow.
How can I retrieve these differences faster without altering the table?

Answer 1

You cannot. This is the fastest query possible, although it may become a lot faster if you add an index on Session , Prev and Id .