Need to get the value from a column whose column name is based on a value in another table - sql

Question

Table A has columns ID, COL1, COL2, COL3.
Table B has columns AID, ColumnName.
I need to get the [ColumnName] value in Table A based on the value of [ColumnName] in Table B.
In the example below:
For ID 1, I need to get the value of column COL1 (This is the value of [ColumnName] for AID 1 in Table B).
For ID 2, I need to get the value of column COL3 (This is the value of [ColumnName] for AID 2 in Table B).
Table A
ID COL1 COL2 COL3
1 a aa aaa
2 b bb bbb
Table B
AID ColumnName
1 COL1
2 COL3
Desired Result:
ID VALUE
1 a
2 bbb
How can I do that ?
Thank you.

Answer 1

Unpivot then join
drop table t
go
drop table t1
go
create table t
(ID int, COL1 varchar(10), COL2 varchar(10), COL3 varchar(10))
go
create table t1
(AID int,ColumnName varchar(10));
go
insert into t values
(1 , 'a', 'aa', 'aaa'),
(2 , 'b', 'bb', 'bbb')
go
insert into t1 values
(1 , 'COL1'),
(2 , 'COL3')
go
with cte as
(select id, u.col, u.val
from t
unpivot
(
val
for col in (col1, col2, col3)
) u
)
select cte.id,cte.val
from cte
join t1 on
t1.aid = cte.id and
t1.columnname = cte.col
go
id val
----------- ----------
1 a
2 bbb
(2 row(s) affected)

Answer 2

One possible approach is to unpivot the columns in TableA using VALUES table value constructor and additional APPLY operator:
Tables:
SELECT *
INTO TableA
FROM (VALUES
(1, 'a', 'aa', 'aaa'),
(2, 'b', 'bb', 'bbb')
) v (ID, COL1, COL2, COL3)
SELECT *
INTO TableB
FROM (VALUES
(1, 'COL1'),
(2, 'COL3')
) v (AID, COL)
Statement:
SELECT b.AID, v.VALUE
FROM TableB b
JOIN TableA a ON b.AID = a.ID
CROSS APPLY (VALUES
('COL1', a.COL1),
('COL2', a.COL2),
('COL3', a.COL3)
) v (COL, [VALUE])
WHERE v.COL = b.COL
Result:
AID
VALUE
1
a
2
bbb