Dynamically Select Column In SQL

Question

The field I want to pull depends on the value of another field. I have a lookup table that looks like this:

Prefix	A_value	B_value	C_value	D_value
A	1	2	3	4
B	5	6	7	8
C	9	10	11	12

I am looking for a way to pull the appropriate value based on the Prefix column. Would be something like this:

Select selected_column = Prefix + '_value'

Where the desired result is:

selected_column
1
6
11

I could easily achieve this with a bunch of CASE/WHEN expressions, but I wanted to see if there's a way to easily call the desired column dynamically.

Answer 1

Well, if it really is A_value -> Z_value , you can use CHOOSE :

SELECT selected_column = CHOOSE
                         (
                           ASCII(Prefix)-64, 
                           A_value, 
                           B_value, 
                           C_value,
                           D_value
                         ) 
FROM dbo.[a lookup table];

• Example db

Answer 2

You could go the JSON route.

 select json_value(json_values, '$[0].'+Prefix+'_value') as selected_column from your_lookup_table t cross apply ( select t.* for json path ) ca(json_values)

selected_column
1
6
11

db<>fiddle here

Answer 3

You can use CURSOR:

declare @t table (Prefix nvarchar(50),A_value int,B_value int,C_value int,D_value int)

insert into @t values ('A', 1,  2,  3,  4)
insert into @t values ('B', 5,  6,  7,  8)
insert into @t values ('C', 9,  10, 11, 12)

declare @result table (selected_column int)
declare @Prefix nvarchar(50),@A_value int,@B_value int,@C_value int,@D_value int

DECLARE db_cursor CURSOR FOR 
SELECT Prefix,A_value,B_value, C_value,D_value
FROM @t 
OPEN db_cursor
FETCH NEXT FROM db_cursor INTO @Prefix,@A_value,@B_value, @C_value,@D_value  

WHILE @@FETCH_STATUS = 0  
 
BEGIN  
    insert into @result
    Select CASE
        WHEN @Prefix='A' THEN @A_value
        WHEN @Prefix='B' THEN @B_value
        WHEN @Prefix='C' THEN @C_value
        WHEN @Prefix='D' THEN @D_value
    END;
    FETCH NEXT FROM db_cursor INTO @Prefix,@A_value,@B_value, @C_value,@D_value 
END 

CLOSE db_cursor  
DEALLOCATE db_cursor 

select * from @result