How to join data using dynamic table and field names - sql

Question

So I have some tables that contains a foreign key that refers to different tables and fields depending on the context. I have a key source table that contains the relevant table and field names. Then I need to be able to lookup the values in those tables/fields.
I simplified it to the following tables/data for demonstration of what I have:
CREATE TABLE [dbo].[WorkOrderStatus](
[WorkOrderStatusId] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](60) NOT NULL
) ON [PRIMARY]
CREATE TABLE [dbo].[Note](
[NoteId] [int] IDENTITY(1,1) NOT NULL,
[NoteKeySourceId] [int] NULL,
[KeyValue] [int] NULL,
[Note] [nvarchar](max) NOT NULL,
) ON [PRIMARY]
CREATE TABLE [dbo].[NoteKeySource](
[NoteKeySourceId] [int] IDENTITY(1,1) NOT NULL,
[SourceTable] [nvarchar](60) NOT NULL,
[SourceField] [nvarchar](60) NOT NULL,
[SourceTextField] [nvarchar](60) NOT NULL,
) ON [PRIMARY]
GO
INSERT INTO WorkOrderStatus VALUES('Open')
DECLARE #OpenStatusId int = SCOPE_IDENTITY();
INSERT INTO WorkOrderStatus VALUES('Closed')
DECLARE #ClosedStatusId int = SCOPE_IDENTITY();
INSERT INTO NoteKeySource VALUES('WorkOrderStatus', 'WorkOrderStatusId', 'Name')
DECLARE #WorkOrderSource int = SCOPE_IDENTITY();
INSERT INTO Note VALUES(#WorkOrderSource, #OpenStatusId, 'Opened the work order')
INSERT INTO Note VALUES(#WorkOrderSource, #ClosedStatusId, 'Closed the work order')
So I need to be able to query a list of Notes, including the SourceTextField referred to in NoteKeySource, identified by KeyValue.
Here is the SP that I have now, which works:
CREATE PROCEDURE [dbo].[spGetNotes]
AS
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
CREATE TABLE #NoteTable (
NoteId int NOT NULL,
SourceTable nvarchar(60),
SourceField nvarchar(60),
SourceTextField nvarchar(60),
KeyValue int,
Context nvarchar(MAX),
Note nvarchar(MAX),
)
INSERT INTO #NoteTable (NoteId, SourceTable, SourceField, SourceTextField, KeyValue, Note)
SELECT N.NoteId, NKS.SourceTable, NKS.SourceField, NKS.SourceTextField, N.KeyValue, N.Note
FROM Note AS N
LEFT OUTER JOIN NoteKeySource AS NKS
ON NKS.NoteKeySourceId = N.NoteKeySourceId
DECLARE #Context nvarchar(max)
DECLARE #Sql nvarchar(255);
DECLARE #SqlDecl nvarchar(255) = N'#Key int, #Ctx nvarchar(MAX) OUTPUT';
DECLARE #NoteId int, #KeyValue int, #SourceTable nvarchar(60), #SourceField nvarchar(60), #SourceTextField nvarchar(60)
DECLARE db_cursor CURSOR FOR SELECT SourceTable, SourceField, SourceTextField, KeyValue
FROM #NoteTable WHERE SourceTable IS NOT NULL FOR UPDATE OF Context
OPEN db_cursor
FETCH NEXT FROM db_cursor INTO #SourceTable, #SourceField, #SourceTextField, #KeyValue
WHILE ##FETCH_STATUS = 0
BEGIN
SET #Sql = N'SELECT #Ctx = ' + #SourceTextField + ' FROM ' + #SourceTable + ' WHERE ' + #SourceField + '= #Key';
EXECUTE sp_executesql #Sql, #SqlDecl, #Key = #KeyValue, #Ctx = #Context OUTPUT
UPDATE #NoteTable SET Context=#Context WHERE CURRENT OF db_cursor
FETCH NEXT FROM db_cursor INTO #SourceTable, #SourceField, #SourceTextField, #KeyValue
END
CLOSE db_cursor
DEALLOCATE db_cursor
SELECT NoteId, Context, Note from #NoteTable
DROP TABLE #NoteTable
RETURN 0
Calling spGetNotes returns:
1:Open:Opened the work order
2:Closed:Closed the work order
It works, but it is pretty ugly, and I would just like to know if there is a better/cleaner/more appropriate way to do this without using a cursor or a while loop. It is my understanding that dynamic SQL cannot be used in a subquery of normal SQL, so this was the best I could come up with.
I am relatively green when it comes to SQL, so if I am wrong about that, please enlighten me! Also, if there is a better overall approach to the general domain problem, that would also be welcome and appreciated :)

Answer 1

Create a view that provides a single EAV overlay across your source tables. Then join NoteKeySource to it.
CREATE VIEW AllSources AS (
SELECT
'SourceTable1' AS [SourceTable], [SourceField], [SourceTextField]
FROM SourceTable1
UNPIVOT([SourceTextField] FOR [SourceField] IN
('Table1Field1','Table1Field2','Table1Field3','Table1Field4')
) p
UNION ALL
SELECT
'SourceTable2' AS [SourceTable], [SourceField], [SourceTextField]
FROM SourceTable2
UNPIVOT([SourceTextField] FOR [SourceField] IN
('Table2Field1','Table2Field2')
) p
UNION ALL
SELECT
'SourceTable3' AS [SourceTable], [SourceField], [SourceTextField]
FROM SourceTable3
UNPIVOT([SourceTextField] FOR [SourceField] IN
('Table3Field1','Table3Field2','Table3Field3')
) p
--etc
)