SQL Server 2017

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CREATE TABLE Import_Table
(
  import_id INT IDENTITY,
  Test1 decimal(5,2),
  Test2 decimal(5,2), 
  Test3 decimal(7,3)
)

CREATE TABLE Fields
(
  FieldName VARCHAR(255)
)
INSERT INTO Import_Table
VALUES (323,2.3,4.5)

INSERT INTO Fields
VALUES ('Test1'),
  ('Test2'),
  ('Test3')
4 rows affected

SELECT Import_id, 
  p.Field, 
  p.Value
FROM
(
  SELECT Import_id,
    CAST(Test1 AS VARCHAR) AS Test1, 
    CAST(Test2 AS VARCHAR) AS Test2, 
    CAST(Test3 AS VARCHAR) AS Test3
  FROM Import_Table
) src
UNPIVOT
(
  Value FOR Field IN (Test1, Test2, Test3)
) p


Import_id
Field
Value


1
Test1
323.00

1
Test2
2.30

1
Test3
4.500


DECLARE @Sql NVARCHAR(MAX)

SET @Sql = '
SELECT Import_id, 
  p.Field, 
  p.Value
FROM
(
  SELECT Import_id,
    '

SELECT @Sql = @Sql + 
  STRING_AGG('CAST(' + FieldName + ' AS VARCHAR) AS ' + FieldName, ', ')
FROM Fields

SET @Sql = @Sql + '
  FROM Import_Table
) src
UNPIVOT
(
  Value FOR Field IN ('

SELECT @Sql = @Sql + 
  STRING_AGG(FieldName, ', ')
FROM Fields
SET @Sql = @Sql + ')
) p'


EXEC sp_executesql @Sql


Import_id
Field
Value


1
Test1
323.00

1
Test2
2.30

1
Test3
4.500