Dynamically selecting the column to select from the row itself in SQL - sql

Question

I have a SQL Server table with some data as follows. The number of P columns are fixed but there will be too many columns. There will be multiple columns in the fashion like S1, S2 etc
Id
SelectedP
P1
P2
P3
P4
P5
1
P2
3
8
4
15
7
2
P1
0
2
6
0
3
3
P3
1
15
2
1
11
4
P4
3
4
6
2
4
I need to write a SQL statement which can get the below result. Basically which column that needs to be selected from each row depends upon the SelectedP value in that row itself. The SelectedP contains the column to select for each row.
Id
SelectedP
Selected-P-Value
1
P2
8
2
P1
0
3
P3
2
4
P4
2
Thanks in advance.

Answer 1

You just need a CASE expression...
SELECT
id,
SelectedP,
CASE SelectedP
WHEN 'P1' THEN P1
WHEN 'P2' THEN P2
WHEN 'P3' THEN P3
WHEN 'P4' THEN P4
WHEN 'P5' THEN P5
END
AS SelectedPValue
FROM
yourTable
This will return NULL for anything not mentioned in the CASE expression.
EDIT:
An option with just a little less typing...
SELECT
id, SelectedP, val
FROM
yourTable AS pvt
UNPIVOT
(
val FOR P IN
(
P1,
P2,
P3,
P4,
P5
)
)
AS unpvt
WHERE
SelectedP = P
NOTE: If the value of SelectedP doesn't exist in the UNPIVOT, then the row will not appear at all (unlike the CASE expression which will return a NULL)
Demo: https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=b693738aac0b594cf37410ee5cb15cf5
EDIT 2:
I don't know if this will perform much worse than the 2nd option, but this preserves the NULL behaviour.
(The preferred option is still to fix your data-structure.)
SELECT
id, SelectedP, MAX(CASE WHEN SelectedP = P THEN val END) AS val
FROM
yourTable AS pvt
UNPIVOT
(
val FOR P IN
(
P1,
P2,
P3,
P4,
P5
)
)
AS unpvt
GROUP BY
id, SelectedP
Demo : https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=f3f64d2fb6e11fd24d1addbe1e50f020