SQL Server-将表连接到自身时无效的对象名

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 --

Answer 2

I think your select should be of the form:

SELECT * 
FROM [table] t1
LEFT JOIN [table] t2 ON (your conditions)

From a performance perspective, this is identical to your last select and to the CTE solution in Mahmoud's answer (I've reviewed the execution plan for all three in SQL Server).

It might only be a matter of taste, but I find this form to be more readable/maintainable.