Question

I’ve tried to find an answer to my problem but couldn’t find similar example.

I have results from such a query

SELECT * FROM (
   SELECT id FROM table
) AS t1

Now I would like to join t1 to another instance of itself because I need to shift it. For example if I wanted to compare a row with the previous one. I tried:

SELECT * FROM (
   SELECT id FROM table
) AS t1
LEFT JOIN t1 AS t2 ON (my conditions)

But I get an error that t1 is invalid object name. When I copy my select statement:

SELECT * FROM (
   SELECT id FROM table
) AS t1
LEFT JOIN (
   SELECT id FROM table
) AS t2 ON (my conditions)

The above works, but is it not slower than joining to already returned results?

Any help would be appreciated

Answer 1

The first one is in correct:

SELECT * FROM (
   SELECT id FROM table
) AS t1
LEFT JOIN t1 AS t2 ON (my conditions)

Because you can’t alias an alias. You can do something similar to it using CTE like so:

;WITH cte
AS
(
    SELECT * FROM Table   
)
SELECT *
FROM Cte t1
INNER JOIN cte t2 ON --