How to display same id of row in same id of 1st row?

Question

Below output:

   Reg_id          class_id                  reg_date      
 2011213807261  0000111016294       2019-02-14 12:22:24.057
 2011213807254  0000111016294       2019-02-14 12:22:24.057
 2011214239559  0000111016447       2019-09-21 03:55:25.357
 2011214239573  0000111016447       2019-09-21 03:55:25.357
 4511999718167  0000111016455       2017-07-23 10:39:36.043
 4511999718235  0000111016455       2017-07-23 10:39:36.043
 4511999717931  0000111016456       2017-03-17 12:35:32.537
 4511999717924  0000111016456       2017-03-17 12:35:32.537

Above output I have same class id of two records. I want 2nd row of same class id as 1st row put in same id of 1st row.

Needed output

 Reg_id            class_id          reg_date             Reg_id            class_id          reg_date 
2011213807261   0000111016294    2019-02-14 12:22:24.057  2011213807254  0000111016294   2019-02-14 12:22:24.057
2011214239559   0000111016447    2019-09-21 03:55:25.357  2011214239573  0000111016447   2019-09-21 03:55:25.357
4511999718167   0000111016455    2017-07-23 10:39:36.043  4511999718235  0000111016455   2017-07-23 10:39:36.043
4511999717931   0000111016456    2017-03-17 12:35:32.537  4511999717924  0000111016456   2017-03-17 12:35:32.537

I need output like above, same id records should display in front of same id of 1st record row.

Answer 1

You can achieve this using ROW_NUMBER() function as given below:

declare @TABLE Table(reg_id varchar(20), class_id varchar(20), regdate datetime)

insert into @table values
('2011213807261','0000111016294','2019-02-14T12:22:24.057')
,('2011213807254','0000111016294','2019-02-14T12:22:24.057');

WITH CTE_TableRank AS 
(
    SELECT *,ROW_NUMBER() OVER (PARTITION BY class_id ORDER BY reg_id) as rnk
    FROM @table
)
SELECT c1.reg_id, c1.class_id, c1.regdate, c2.reg_id, c2.class_id, c2.regdate
FROM CTE_TableRank AS c1 
INNER JOIN CTE_TableRank AS c2 ON c1.class_id = c1.class_id
WHERE c1.rnk = 1 AND c2.rnk = 2

reg_id	class_id	regdate	reg_id	class_id	regdate
2011213807254	0000111016294	2019-02-14 12:22:24.057	2011213807261	0000111016294	2019-02-14 12:22:24.057