How do I compare 2 rows from the same table (SQL Server)?

Question

I need to create a background job that processes a table looking for rows matching on a particular id with different statuses. It will store the row data in a string to compare the data against a row with a matching id.

I know the syntax to get the row data, but I have never tried comparing 2 rows from the same table before. How is it done? Would I need to use variables to store the data from each? Or some other way?

(Using SQL Server 2008)

Answer 1

You can join a table to itself as many times as you require, it is called a self join.

An alias is assigned to each instance of the table (as in the example below) to differentiate one from another.

SELECT a.SelfJoinTableID
FROM   dbo.SelfJoinTable a
       INNER JOIN dbo.SelfJoinTable b
         ON a.SelfJoinTableID = b.SelfJoinTableID
       INNER JOIN dbo.SelfJoinTable c
         ON a.SelfJoinTableID = c.SelfJoinTableID
WHERE  a.Status = 'Status to filter a'
       AND b.Status = 'Status to filter b'
       AND c.Status = 'Status to filter c' 

Answer 2

OK, after 2 years it's finally time to correct the syntax:

SELECT  t1.value, t2.value
FROM    MyTable t1
JOIN    MyTable t2
ON      t1.id = t2.id
WHERE   t1.id = @id
        AND t1.status = @status1
        AND t2.status = @status2