SQL Server 2019

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CREATE TABLE yourTable (
  id         INT,
  SelectedP  VARCHAR(4),
  P1         INT,
  P2         INT,
  P3         INT,
  P4         INT,
  P5         INT
);
INSERT INTO yourTable VALUES (1,'P2',4,3,5,4,4);  
INSERT INTO yourTable VALUES (2,'P2',4,1,5,5,5);  
INSERT INTO yourTable VALUES (3,'P3',4,3,5,4,4);  
INSERT INTO yourTable VALUES (4,'P3',4,2,5,5,4);  
INSERT INTO yourTable VALUES (5,'P4',5,1,5,5,5);  
5 rows affected

SELECT
  id, SelectedP, val  
FROM   
   yourTable AS pvt
UNPIVOT  
(
  val FOR P IN   
  (
    P1,
    P2,
    P3,
    P4,
    P5
  )
)
  AS unpvt
WHERE
  SelectedP = P


id
SelectedP
val


1
P2
3

2
P2
1

3
P3
5

4
P3
5

5
P4
5