Compare two tables using field map - sql-server

Question

I need to compare the values between tables in two SQL Server databases. The fieldnames in the tables in one database don't match the fieldnames in the tables in the second database. I have a link table that has the matching table names and the matching field names mapped.
Table1:
| Tab1_ID | Field1 | Field2 | Field3 |
|---------|--------|--------|--------|
| 1 | One | Two | Three |
| 2 | Two | Two | One |
| 3 | Three | Two | Two |
| 4 | Two | One | One |
Table2:
| Tab2_ID | Field_1 | Field_2 | Field_3 |
|---------|---------|---------|---------|
| 1 | One | Two | Three |
| 2 | Two | Five | One |
| 3 | Three | Two | Two |
| 4 | Two | One | Six |
Link Table:
| LinkTab_ID | Tab1 | Tab2 | Tab1Fld | Tab2Fld |
|------------|--------|--------|---------|--------------|
| 100 | Table1 | Table2 | Field1 | Field_1 |
| 105 | Table1 | Table2 | Field2 | Field_2 |
| 110 | Table1 | Table2 | Field3 | Field_3 |
| 124 | Table1 | Table4 | Field1 | Fieldname_01 |
| 166 | Table3 | Table5 | F3 | FN_3 |
Is it possible to use the Link Table to somehow specify the field names to compare between the two tables?
typically I'd do something like
SELECT
*
FROM
Table1 INNER JOIN Table2 ON Tab1_ID = Tab2_ID
WHERE
Table1.Field1 != Table2.Field_1
OR Table1.Field2 != Table2.Field_2
However I have many tables and many fields and the fieldnames change (i.e. new fields). My one constant is that the two are mapped in the link table.
The tables are one-to-one and fields are one-to-one.

Answer 1

This is only an approach you will need to expand it to suit. In particular you need a method for handling the join predicate(s).
also see this SQL Fiddle
CREATE TABLE LinkTable
([LinkTab_ID] int, [Tab1] varchar(6), [Tab2] varchar(6), [Tab1Fld] varchar(6), [Tab2Fld] varchar(12))
;
INSERT INTO LinkTable
([LinkTab_ID], [Tab1], [Tab2], [Tab1Fld], [Tab2Fld])
VALUES
(100, 'Table1', 'Table2', 'Field1', 'Field_1'),
(105, 'Table1', 'Table2', 'Field2', 'Field_2'),
(110, 'Table1', 'Table2', 'Field3', 'Field_3'),
(124, 'Table1', 'Table4', 'Field1', 'Fieldname_01'),
(166, 'Table3', 'Table5', 'F3', 'FN_3')
;
CREATE TABLE Table1
([Tab1_ID] int, [Field1] varchar(5), [Field2] varchar(3), [Field3] varchar(5))
;
INSERT INTO Table1
([Tab1_ID], [Field1], [Field2], [Field3])
VALUES
(1, 'One', 'Two', 'Three'),
(2, 'Two', 'Two', 'One'),
(3, 'Three', 'Two', 'Two'),
(4, 'Two', 'One', 'One')
;
CREATE TABLE Table2
([Tab2_ID] int, [Field_1] varchar(9), [Field_2] varchar(9), [Field_3] varchar(9))
;
INSERT INTO Table2
([Tab2_ID], [Field_1], [Field_2], [Field_3])
VALUES
('1', 'One', 'Two', 'Three'),
('2', 'Two', 'Five', 'One'),
('3', 'Three', 'Two', 'Two'),
('4', 'Two', 'One', 'Six')
;
Query 1:
DECLARE #t1 AS NVARCHAR(30) = 'Table1'
DECLARE #t2 AS NVARCHAR(30) = 'Table2'
DECLARE #filter AS NVARCHAR(MAX)
DECLARE #query AS NVARCHAR(MAX)
SET #filter = STUFF((SELECT ' OR ' + concat(Tab1, '.', Tab1Fld, ' <> ', Tab2, '.', Tab2Fld)
FROM LinkTable
WHERE Tab1 = #t1 AND Tab2 = #t2
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,4,'')
SET #query = 'SELECT * FROM '
+ #t1
+ ' INNER JOIN '
+ #t2
+ ' ON '
+ #t1
+ '.Tab1_ID = '
+ #t2
+ '.Tab2_ID'
+ ' WHERE '+ #filter
select #query
--execute(#query)
Results:
| SELECT * FROM Table1 INNER JOIN Table2 ON Table1.Tab1_ID = Table2.Tab2_ID WHERE Table1.Field1 <> Table2.Field_1 OR Table1.Field2 <> Table2.Field_2 OR Table1.Field3 <> Table2.Field_3 |