Retrieve data from table2 using column names stored in table1 - sql-server

Question

I want to be able to look up fields values from a secondary table that stores the secondary table field names as strings in a primary table.
I am trying to both query the primary table itself to get the field names I want from the secondary table:
How can I use that string that contains the field name to select data from table2, given the id of a specific row.
The use case here, is that table1 also contains a note field. I want to populate that note field with the contents of a specific record in table2. I want to do this for all the records in table 1: each record references a different field by field name.
Another way to look at this:
Table2 has 1 row we want data from, and we know the ID
table1 stores the list of fields that we need data for from table 2
How do I get both pieces of data (field name, and field value) using these constraints?
all data is varchar
table1
fieldname
externalID
myField1
001
myField2
001
table2
id
myField1
myField2
myField3
001
myField1ValueForID001
myField2ValueForID001
myField3ValueForID001
002
moredata1
moredata2
moredata3
select fieldname,
(select [fieldname] from table2 where id = ) as fieldData
from table1
result
fieldname
fieldData
myField1
myField1
myField2
myField2
desired result
fieldname
fieldData
myField1
myField1ValueForID001
myField2
myField2ValueForID001

Answer 1

You can do this with a CASE expression, though I do recommend you rethink your design here:
SELECT *
INTO dbo.Table1
FROM (VALUES('myField1','001'),
('myField2','001'))V(FieldName,ExternalID);
SELECT *
INTO dbo.Table2
FROM (VALUES('001','myField1ValueForID001','myField2ValueForID001','myField3ValueForID001'),
('002','moredata1','moredata2','moredata3'))V(id,myField1,myField2,myField3);
GO
SELECT T1.FieldName,
CASE T1.FieldName WHEN 'myField1' THEN T2.myField1
WHEN 'myField2' THEN T2.myField2
WHEN 'myField3' THEN T2.myField3
END AS FieldData
FROM dbo.Table1 T1
JOIN dbo.Table2 T2 ON T1.ExternalID = T2.id;
GO
DROP TABLE dbo.Table1;
DROP TABLE dbo.Table2;