Comparing Column Values in the Same Table

The Set-Up:

This is yet another time that a blog post has come about from a question by a developer. They’re good guys, I guess, they keep me on my toes.

This time it was with change logging. We didn’t have Change Data Capture (CDC), or Temporal Tables enabled (have you seen the YouTube videos by Bert Wagner ( blog | twitter ) on these?). What we did have was “manual logging” and no, I’m not even talking about Triggers.

What we had was INSERT statements, directly after a MERGE statement, that inserted into a table variable a hard-coded name of the column, the old value, and the new value.

Is that what I would do? Doesn’t matter, it was there before I got there, seems to work, and is low down on the list of priorities to change.

The question was, every time that they needed to add a column to a table, and change log it, they had to add multiple lines to the change tracking procedure and the procedure was getting gross and hard to maintain.

Something to do with DRYness?

Create Table:

You know the drill by now, I quite like to play along so let us facilitate that (from now on I’m going to use Gist, formatting with native WordPress is starting to annoy me).

	USE Pantheon;
	GO

	DROP TABLE IF EXISTS dbo.GotAnyChange;

	CREATE TABLE dbo.GotAnyChange
	(
	GotAnyChangeID int IDENTITY(1, 1),
	Column1 int,
	Column2 char(1),
	Column3 date,
	Column4 bigint,
	Column5 varchar(50),
	Column6 datetime
	);

	INSERT INTO dbo.GotAnyChange
	( Column1,
	Column2,
	Column3,
	Column4,
	Column5,
	Column6
	)
	VALUES
	(0, — Column1 – int
	'A', — Column2 – char(1)
	GETDATE(), — Column3 – date
	0, — Column4 – bigint
	REPLICATE('A', 50), — Column5 – varchar(50)
	GETDATE() — Column6 – datetime
	);

	SELECT *
	FROM dbo.GotAnyChange;

view raw

CreateGotAnyChange.sql

hosted with ❤ by GitHub

This will create our table and, luckily, all of it’s columns are important enough to warrant capturing when they get changed.

FirstCreation — Despite their looks, these values are “important”

Old, Way WHERE old=way

Let’s take a look at the code that they were using, shall we?

	USE Pantheon;
	GO

	DECLARE @Id int = 1

	— ChangeLog data store
	— Yes, I know that they're all varchars…
	DECLARE @ChangeLogTemp table
	(
	ColumnName varchar(100),
	OldValue varchar(20),
	NewValue varchar(20)
	);

	— Old & New value data store
	DECLARE @OutputTemp table
	(
	OldColumn1 int NOT NULL,
	NewColumn1 int NOT NULL,
	OldColumn2 char(1) NOT NULL,
	NewColumn2 char(1) NOT NULL,
	OldColumn3 date NOT NULL,
	NewColumn3 date NOT NULL,
	OldColumn4 bigint NULL,
	NewColumn4 bigint NULL,
	OldColumn5 varchar(50) NOT NULL,
	NewColumn5 varchar(50) NOT NULL,
	OldColumn6 datetime NULL,
	NewColumn6 datetime NULL
	);

	— Actual update, outputting the old and new values
	UPDATE gac
	SET gac.Column1 = 1,
	gac.Column2 = 'B',
	gac.Column6 = GETDATE()
	OUTPUT Deleted.Column1, Inserted.Column1,
	Deleted.Column2, Inserted.Column2,
	Deleted.Column3, Inserted.Column3,
	Deleted.Column4, Inserted.Column4,
	Deleted.Column5, Inserted.Column5,
	Deleted.Column6, Inserted.Column6
	INTO @OutputTemp
	FROM dbo.GotAnyChange AS gac
	WHERE gac.GotAnyChangeID = @Id;

	— Capture changes. 1 for each column we want to capture
	INSERT INTO @ChangeLogTemp
	SELECT 'Column1',
	OldColumn1,
	NewColumn1
	FROM @OutputTemp
	WHERE ISNULL(OldColumn1, NewColumn1) <> NewColumn1;

	INSERT INTO @ChangeLogTemp
	SELECT 'Column2',
	OldColumn2,
	NewColumn2
	FROM @OutputTemp
	WHERE ISNULL(OldColumn2, NewColumn2) <> NewColumn2;

	INSERT INTO @ChangeLogTemp
	SELECT 'Column3',
	OldColumn3,
	NewColumn3
	FROM @OutputTemp
	WHERE ISNULL(OldColumn3, NewColumn3) <> NewColumn3;

	INSERT INTO @ChangeLogTemp
	SELECT 'Column4',
	OldColumn4,
	NewColumn4
	FROM @OutputTemp
	WHERE ISNULL(OldColumn4, NewColumn4) <> NewColumn4;

	INSERT INTO @ChangeLogTemp
	SELECT 'Column5',
	OldColumn5,
	NewColumn5
	FROM @OutputTemp
	WHERE ISNULL(OldColumn5, NewColumn5) <> NewColumn5;

	INSERT INTO @ChangeLogTemp
	SELECT 'Column6',
	OldColumn6,
	NewColumn6
	FROM @OutputTemp
	WHERE ISNULL(OldColumn6, NewColumn6) <> NewColumn6;

	— Capture all changes as an XML
	SELECT ColumnName AS CN,
	OldValue AS OV,
	NewValue AS NV
	FROM @ChangeLogTemp
	FOR XML PATH('Change'), ROOT('Changes');

view raw

OldWayChangeCapture.sql

hosted with ❤ by GitHub

And the results?

OldWayResult — XML anyone?

You can probably see the problem here.

Hey! It’s legacy code, let’s focus on just 1 problem at at time!

The main issue that I was asked about was every time a column was deemed important and needed to be added to the list, they had to insert another INSERT INTO @ChangeLogTemp... and they thought that it wasn’t sustainable in the long run.

Hmmm it also comes across as very RBAR doesn’t it? Every time we want to include another column to the change tracking, we have to add them row by agonizing row. The script is already big enough, if we keep adding more, it will get massive!

Set based is 90% of the time the right way to go but how do we do set based solutions on the same table?

New JOIN Way ON new = way

The first thing I do is to change that table variable into a temp table. Stats, indexes (if necessary), and I can query the results as we go along. Much better!

ChangeToTempTable — Temp > Variable?

The second thing is that, whether by luck or by design, the legacy code has the same naming conventions for the columns; new column values are have the prefix “New%” in the column name and old columns have the “Old%” prefix.
This works for us because we can now split the new columns into 2 derived tables, New and Old, and that way we have the differences.

PreUnPivotColumns — Potential problem here…

Have you ever tried to find the differences between two consecutive rows of data? It’s fiendishly difficult. WHERE Column1 on row1 != Column1 on row2 apparently just does not work, le sigh.

I’ve talked before about PIVOT but now I’m going to introduce you to it’s little brother, UNPIVOT, which “rotating columns of a table-valued expression into column values”

I say “little brother” because the whole document talks about PIVOT, with only brief mentions of UNPIVOT in the notes.

If you’re writing documentation like this, please stop.

With UNPIVOT we can create a table of our rows around our ID and Column names…

UnpivotedColumns — Potential problem averted!

… and with this, we can join on our ID and Column names and get to our more intuitive WHERE OldValue != NewValue.

Bringing it all together!

	USE Pantheon;
	GO

	DECLARE @ChangeLogAsXml xml,
	@Id int = 1;

	DROP TABLE IF EXISTS #OutputTableNew;
	CREATE TABLE #OutputTableNew
	(
	GotAnyChangeID int NOT NULL,
	OldColumn1 int NOT NULL,
	NewColumn1 int NOT NULL,
	OldColumn2 char(1) NOT NULL,
	NewColumn2 char(1) NOT NULL,
	OldColumn3 date NOT NULL,
	NewColumn3 date NOT NULL,
	OldColumn4 bigint NULL,
	NewColumn4 bigint NULL,
	OldColumn5 varchar(50) NOT NULL,
	NewColumn5 varchar(50) NOT NULL,
	OldColumn6 datetime NULL,
	NewColumn6 datetime NULL
	);

	UPDATE gac
	SET gac.Column1 = 2,
	gac.Column2 = 'C',
	gac.Column6 = GETDATE()
	OUTPUT inserted.GotAnyChangeID,
	Deleted.Column1, Inserted.Column1,
	Deleted.Column2, Inserted.Column2,
	Deleted.Column3, Inserted.Column3,
	Deleted.Column4, Inserted.Column4,
	Deleted.Column5, Inserted.Column5,
	Deleted.Column6, Inserted.Column6
	INTO #OutputTableNew
	FROM dbo.GotAnyChange AS gac
	WHERE gac.GotAnyChangeID = @Id;

	SELECT New.ColumnName AS CN,
	New.NewValue AS NV,
	Old.OldValue AS OV
	FROM
	(
	SELECT Unpvt.GotAnyChangeID,
	Unpvt.ColumnName,
	Unpvt.NewValue
	FROM
	( SELECT [GotAnyChangeID],
	CAST(ISNULL([NewColumn1], '') AS nvarchar(20)) AS [Column1],
	CAST(ISNULL([NewColumn2], '') AS nvarchar(20)) AS [Column2],
	CAST(ISNULL([NewColumn3], '') AS nvarchar(20)) AS [Column3],
	CAST(ISNULL([NewColumn4], '') AS nvarchar(20)) AS [Column4],
	CAST(ISNULL([NewColumn5], '') AS nvarchar(20)) AS [Column5],
	CAST(ISNULL([NewColumn6], '') AS nvarchar(20)) AS [Column6]
	FROM #OutputTableNew
	) AS DataSource UNPIVOT(NewValue FOR ColumnName IN([Column1], [Column2], [Column3], [Column4], [Column5], [Column6])) AS Unpvt
	) AS New
	INNER JOIN
	(
	SELECT Unpvt.GotAnyChangeID,
	Unpvt.ColumnName,
	Unpvt.OldValue
	FROM
	( SELECT [GotAnyChangeID],
	CAST(ISNULL([OldColumn1], '') AS nvarchar(20)) AS [Column1],
	CAST(ISNULL([OldColumn2], '') AS nvarchar(20)) AS [Column2],
	CAST(ISNULL([OldColumn3], '') AS nvarchar(20)) AS [Column3],
	CAST(ISNULL([OldColumn4], '') AS nvarchar(20)) AS [Column4],
	CAST(ISNULL([OldColumn5], '') AS nvarchar(20)) AS [Column5],
	CAST(ISNULL([OldColumn6], '') AS nvarchar(20)) AS [Column6]
	FROM #OutputTableNew
	) AS DataSource UNPIVOT(OldValue FOR ColumnName IN([Column1], [Column2], [Column3], [Column4], [Column5], [Column6])) AS Unpvt
	) AS Old
	ON Old.ColumnName = New.ColumnName
	AND Old.GotAnyChangeID = New.GotAnyChangeID
	WHERE New.NewValue <> Old.OldValue
	FOR XML PATH('Change'), ROOT('Changes');

view raw

NewWayChangeCapture.sql

hosted with ❤ by GitHub

And it works!

NewWayResult — wasn’t this replaced by JSON?

It’s not great though.

The whole thing was supposed to be to reduce the amount of changes required when they need to include or exclude columns. All in all though, it’s just 6 lines less. Not exactly the great return that you’d expect.
Yeah, true with the old way for every column we want to add we have to add an extra 6 lines while the new way adds 2.

That means for 1,024 columns:

The old way could have at least 6,144 lines per table. (1024 * 6)
The new way could have at least 2,048 lines per table (not explaining this calculation >:( )

So, is there anything else that we can do?

Dynamically?

I’ve talked before about T-SQL automation with Dynamic SQL and this should be a good candidate for that.

What can we make dynamic here though? How about…

The new and old columns bit?
The FOR ColumnName IN([Column1], [Column2], [Column3], [Column4], [Column5], [Column6]) bit?
The CAST(ISNULL([Old/NewColumn], '') AS nvarchar bit?

Explain it to me.

The new and old columns.

Well, temp tables exist in the tempdb database, they just get a suffix of a lot of underscores and a hex value.

So to get our column names, we can just query the sys.tables and sys.columns catalog views in [tempdb] and we should have what we need.

DynamicColumnsResults — We can add a filter clause too

2. The FOR ColumnName IN (

I’ve talked before about concatenating values so we can use that to generate this part of the script.

DynamicUnpivotColumnNames — LEN(tc.name) – 3 to remove the “old”/”new” prefix

3. The CAST(ISNULL(...

This is basically the same as the above. Don’t be put off by needing to add CAST(ISNULL( before the column names, it’s not as complex as you’d think.

DynamicNewColumnsSelect — STUFF just doesn’t look as pretty… 🙁

Now that we have our dynamic bits, let’s create the full statements.

Full Dynamic Script

	USE Pantheon;
	GO

	DROP TABLE IF EXISTS #OutputTableDynamic;
	GO
	CREATE TABLE #OutputTableDynamic
	(
	GotAnyChangeID int NOT NULL,
	OldColumn1 int NOT NULL,
	NewColumn1 int NOT NULL,
	OldColumn2 char(1) NOT NULL,
	NewColumn2 char(1) NOT NULL,
	OldColumn3 date NOT NULL,
	NewColumn3 date NOT NULL,
	OldColumn4 bigint NULL,
	NewColumn4 bigint NULL,
	OldColumn5 varchar(50) NOT NULL,
	NewColumn5 varchar(50) NOT NULL,
	OldColumn6 datetime NULL,
	NewColumn6 datetime NULL
	);

	DECLARE @ChangeLogAsXml xml,
	@Id int = 1;

	UPDATE gac
	SET gac.Column1 = 3,
	gac.Column2 = 'D',
	gac.Column6 = GETDATE()
	OUTPUT inserted.GotAnyChangeID,
	Deleted.Column1, Inserted.Column1,
	Deleted.Column2, Inserted.Column2,
	Deleted.Column3, Inserted.Column3,
	Deleted.Column4, Inserted.Column4,
	Deleted.Column5, Inserted.Column5,
	Deleted.Column6, Inserted.Column6
	INTO #OutputTableDynamic
	FROM dbo.GotAnyChange AS gac
	WHERE gac.GotAnyChangeID = @Id;

	DECLARE @DynamicSql nvarchar(MAX),
	@NewColumns nvarchar(MAX),
	@OldColumns nvarchar(MAX),
	@columns nvarchar(MAX);

	SET @NewColumns =
	(
	SELECT STUFF(
	(
	SELECT TOP (1024)
	', CAST(ISNULL(' + QUOTENAME(RIGHT(tc.name, LEN(tc.name))) + ', '''') AS nvarchar(20)) AS '
	+ QUOTENAME(RIGHT(tc.name, LEN(tc.name) – 3))
	FROM tempdb.sys.tables AS tt
	INNER JOIN tempdb.sys.columns AS tc
	ON tt.object_id = tc.object_id
	WHERE tt.name LIKE '#OutputTableDynamic%'
	AND tc.name LIKE 'New%'
	ORDER BY tc.column_id
	FOR XML PATH('')
	), 1, 2, ''));
	SET @OldColumns =
	(
	SELECT STUFF(
	(
	SELECT TOP (1024)
	', CAST(ISNULL(' + QUOTENAME(RIGHT(tc.name, LEN(tc.name))) + ', '''') AS nvarchar(20)) AS '
	+ QUOTENAME(RIGHT(tc.name, LEN(tc.name) – 3))
	FROM tempdb.sys.tables AS tt
	INNER JOIN tempdb.sys.columns AS tc
	ON tt.object_id = tc.object_id
	WHERE tt.name LIKE '#OutputTableDynamic%'
	AND tc.name LIKE 'Old%'
	ORDER BY tc.column_id
	FOR XML PATH('')
	), 1, 2, '' ) );
	SET @columns =
	(SELECT STUFF(
	( SELECT TOP (1024)
	', ' + QUOTENAME(RIGHT(tc.name, LEN(tc.name) – 3))
	FROM tempdb.sys.tables AS tt
	INNER JOIN tempdb.sys.columns AS tc
	ON tt.object_id = tc.object_id
	WHERE tt.name LIKE '#OutputTableDynamic%'
	AND tc.name LIKE 'New%'
	ORDER BY tc.column_id
	FOR XML PATH('')
	), 1, 2, ''));
	SET @DynamicSql =
	N'SELECT New.ColumnName AS CN, New.NewValue AS NV, Old.OldValue AS OV FROM (SELECT Unpvt.GotAnyChangeId, Unpvt.ColumnName, Unpvt.NewValue FROM (SELECT [GotAnyChangeId], ' + @NewColumns
	+ N' FROM #OutputTableDynamic ) AS DataSource UNPIVOT (NewValue FOR ColumnName IN (' + @columns
	+ N') ) AS Unpvt ) AS New INNER JOIN ( SELECT Unpvt.GotAnyChangeId, Unpvt.ColumnName, Unpvt.OldValue FROM (SELECT [GotAnyChangeId], ' + @OldColumns
	+ N' FROM #OutputTableDynamic ) AS DataSource UNPIVOT (OldValue FOR ColumnName IN (' + @columns
	+ N')) AS Unpvt) AS Old ON Old.ColumnName = New.ColumnName AND Old.GotAnyChangeId = New.GotAnyChangeId WHERE New.NewValue != Old.OldValue FOR XML PATH(''Change''), ROOT(''Changes'')';

	EXECUTE sp_executesql @DynamicSql;

view raw

DynamicChangeCapture.sql

hosted with ❤ by GitHub

Results are good!

DynamicWayResult — We’ve seen this before

Overall, the script is longer at nearly double the lines but where it shines is when adding new columns.
To include new columns, just add them to the table; to exclude them, just add in a filter clause.

So, potentially, if every column in this table is to be tracked and we add columns all the way up to 1,024 columns, this code will not increase.
Old way: at least 6,144.
New way: at least 2,048.
Dynamic: no change

Summary:

Like the script, this was a massive post. Back at the start, I said that a developer came to me because they wanted to get more DRY (?) and stop needing to add more content to the stored procedure.

Do you think the developer used this?

Nope!

I can’t say that I blame them, it’s slightly ugly and unwieldy, and I wrote it so I should love it.
Yet if something was to go wrong and the need was there to open the procedure and troubleshoot it, the first person to open this up is going to let out a groan of despair!

So this request turned into a proof of concept and nothing more. No skin off my back, I have a growing list of tasks to accomplish by 5 minutes ago. Better get back to them.

Author: Shane O'Neill

DBA, T-SQL and PowerShell admirer, Food, Coffee, Whiskey (not necessarily in that order)... View all posts by Shane O'Neill

3 thoughts on “Comparing Column Values in the Same Table”

natethedba says:

July 12, 2017 at 12:38 am

I had to go thru a similar scenario that ended up being a report of data diffs between two “similar” tables (not “identical” but at least they had matched keys). Sometimes it’s generic enough to dynamically script, sometimes it’s not… I enjoyed your write-up!

Loading...

Reply
1. Shane O'Neill says:
  
  July 12, 2017 at 10:14 am
  
  Thanks!
  I’m actually surprised by the number of people that have to do this. I thought it would not be something that common but I guess I was wrong. Not sure if that is a good thing or a bad thing though…
  
  Loading...
  
  Reply
Pingback: Dynamic Unpivoting For Change Detection – Curated SQL

Leave a ReplyCancel reply