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

Answer 3

Some people find the following alternative syntax easier to see what is going on:

select t1.value,t2.value
from MyTable t1
    inner join MyTable t2 on
        t1.id = t2.id
where t1.id = @id

Answer 4

SELECT COUNT(*) FROM (SELECT * FROM tbl WHERE id=1 UNION SELECT * FROM tbl WHERE id=2) a

如果您有两行，则它们不同，如​​果有一行 - 相同。

Answer 5

SELECT * FROM A AS b INNER JOIN A AS c ON b.a = c.a
WHERE b.a = 'some column value'

Answer 6

I had a situation where I needed to compare each row of a table with the next row to it, (next here is relative to my problem specification) in the example next row is specified using the order by clause inside the row_number() function.

so I wrote this:

DECLARE @T TABLE (col1 nvarchar(50));

insert into @T VALUES ('A'),('B'),('C'),('D'),('E')

select I1.col1 Instance_One_Col, I2.col1 Instance_Two_Col  from (
 select col1,row_number() over (order by col1) as row_num
 FROM @T
) AS I1
left join (
 select col1,row_number() over (order by col1) as row_num
 FROM @T
) AS I2 on I1.row_num = I2.row_num - 1

after that I can compare each row to the next one as I need