SQL Server table audit trigger

Question

I have two tables Customers and AuditTable. When I modify the Customers table, I need to insert a new record into the AuditTable:

CREATE TABLE [dbo].[AuditTable]
(
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [StateBefore] [nvarchar](max) NULL,
    [StateAfter] [nvarchar](max) NULL
)

I need to place an XML representation of Customer state into StateBefore and StateAfter, before and after update.

The Customer table is:

CREATE TABLE [dbo].[Customer]
(
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Name] [nvarchar](256) NOT NULL,
    [Email] [nvarchar](max) NOT NULL,
    [IsDeleted] [bit] NULL,
    [CreatedUtc] [datetime] NOT NULL,
    [UpdatedUtc] [datetime] NULL,
    [Version] [timestamp] NOT NULL,

    CONSTRAINT [PK_dbo.Customer] 
        PRIMARY KEY CLUSTERED ([Id] ASC)
           WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
                 IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON,  
                 ALLOW_PAGE_LOCKS = ON)
)
GO

ALTER TABLE [dbo].[Customer] 
   ADD DEFAULT (getutcdate()) FOR [CreatedUtc]
GO

I found the way how to get an xml representation of rows:

SELECT
    [State] = (SELECT *
               FROM dbo.Customer [Customer]
               WHERE [Customer].Id = cust.Id
               FOR XML AUTO
              )
FROM 
    dbo.Customer cust

It just for example. So in my trigger I need to create the xml representation of rows from deleted and inserted tables.

Here is the trigger:

ALTER TRIGGER [dbo].[UpdateCustomerTrigger]
ON [dbo].[Customer]
FOR UPDATE
AS 
BEGIN
    UPDATE Customer 
    SET UpdatedUtc = GETDATE()
    FROM INSERTED
    WHERE inserted.id = Customer.Id

    -- here I need to insert new records into AuditTable
END

So, how can I join two representations of deleted and inserted tables to correctly insert them into the AuditTable? Thanks.

Gottfried Lesigang · Accepted Answer

Great that you've found a solution already...

I was just thinking about something similar...

Your approach would take two copies of the full record at any tiny change. As I have to deal with tables with a lot of columns, some of them are BLOBs, this would not fit for me.

Well, I did not find an absolutely "clean" approach, but with the following you'll get an AuditLog with only the values which have changed acutally in a better to read style.

Maybe you like it:

Test.scenario

EDIT: Added support for `INSERT` and `DELETE` and `NULL` values.

Try it out:

CREATE TABLE AuditTest(TableSchema VARCHAR(250), TableName VARCHAR(250), AuditType VARCHAR(250),Content XML, LogDate DATETIME DEFAULT GETDATE());
GO

CREATE TABLE dbo.Test(ID INT,Test1 VARCHAR(100),Test2 DATETIME,ModifyCounter INT DEFAULT 0,LastModified DATETIME DEFAULT GETDATE());
INSERT INTO dbo.Test(ID,Test1,Test2) VALUES
 (1,'Test1',{d'2001-01-01'})
,(2,'Test2',{d'2002-02-02'});

--current content

SELECT * FROM dbo.Test;
GO

--The trigger for the audit

CREATE TRIGGER [dbo].[UpdateTestTrigger]
ON [dbo].[Test]
FOR UPDATE,INSERT,DELETE
AS 
BEGIN
   IF NOT EXISTS(SELECT 1 FROM deleted) AND NOT EXISTS(SELECT 1 FROM inserted) RETURN;

   DECLARE @tp VARCHAR(10)=CASE WHEN EXISTS(SELECT 1 FROM deleted) AND EXISTS(SELECT 1 FROM inserted) THEN 'upd'
                           ELSE CASE WHEN EXISTS(SELECT 1 FROM deleted) AND NOT EXISTS(SELECT 1 FROM inserted) THEN 'del' ELSE 'ins' END END;
   WITH UpdateableCTE AS
   (
    SELECT t.LastModified,t.ModifyCounter 
    FROM dbo.Test AS t
    INNER JOIN inserted AS i ON t.ID=i.ID
   )
   UPDATE UpdateableCTE SET LastModified=GETDATE()
                           ,ModifyCounter=ModifyCounter+1;

   SELECT * INTO #tmpInserted FROM inserted;
   SELECT * INTO #tmpDeleted FROM deleted;

   DECLARE @tableSchema VARCHAR(250)='dbo';
   DECLARE @tableName   VARCHAR(250)='Test';

   DECLARE @cols VARCHAR(MAX)=
   STUFF
   (
   (
    SELECT ',' + CASE WHEN @tp='upd' THEN 
           'CASE WHEN (i.[' + COLUMN_NAME + ']!=d.[' + COLUMN_NAME + '] ' +
           'OR (i.[' + COLUMN_NAME + '] IS NULL AND d.[' + COLUMN_NAME + '] IS NOT NULL) ' + 
           'OR (i.['+ COLUMN_NAME + '] IS NOT NULL AND d.[' + COLUMN_NAME + '] IS NULL)) ' +
           'THEN ' ELSE '' END +
           '(SELECT ''' + COLUMN_NAME + ''' AS [@name]' + 
                         CASE WHEN @tp IN ('upd','del') THEN ',ISNULL(CAST(d.[' + COLUMN_NAME + '] AS NVARCHAR(MAX)),N''##NULL##'') AS [@old]' ELSE '' END + 
                         CASE WHEN @tp IN ('ins','upd') THEN ',ISNULL(CAST(i.[' + COLUMN_NAME + '] AS NVARCHAR(MAX)),N''##NULL##'') AS [@new] ' ELSE '' END + 
                  ' FOR XML PATH(''Column''),TYPE) ' + CASE WHEN @tp='upd' THEN 'END' ELSE '' END
    FROM INFORMATION_SCHEMA.COLUMNS
    WHERE TABLE_SCHEMA=@tableSchema AND TABLE_NAME=@tableName
    FOR XML PATH('')
   ),1,1,''
   );

    DECLARE @cmd VARCHAR(MAX)=   
    'SET LANGUAGE ENGLISH;
    WITH ChangedColumns AS
    (
    SELECT COALESCE(i.ID,d.ID) AS ID
            ,Col.*  
    FROM #tmpInserted AS i
    FULL OUTER JOIN #tmpDeleted AS d ON i.ID=d.ID
    CROSS APPLY
    (
        SELECT ' + @cols + ' 
        FOR XML PATH(''''),TYPE
    ) AS Col([Column])
    )
    INSERT INTO AuditTest(TableSchema,TableName,AuditType,Content)
    SELECT ''' + @tableSchema + ''',''' + @tableName + ''',''' + @tp + '''
    ,(
    SELECT ''' + @tableSchema + ''' AS [@TableSchema]
            ,''' + @tableName + ''' AS [@TableName]
            ,''' + @tp + ''' AS [@ActionType]
    ,(
        SELECT ChangedColumns.ID AS [@ID]
        ,(
        SELECT x.[Column] AS [*],''''
        FROM ChangedColumns AS x WHERE x.ID=ChangedColumns.ID
        FOR XML PATH(''''),TYPE
        )
        FROM ChangedColumns
        FOR XML PATH(''Row''),TYPE
        )
    FOR XML PATH(''Changes'')
    );';

    EXEC (@cmd);

   DROP TABLE #tmpInserted;
   DROP TABLE #tmpDeleted;
END
GO

--Now let's test it with some operations:

UPDATE dbo.Test SET Test1='New 1' WHERE ID=1;
UPDATE dbo.Test SET Test1='New 1',Test2={d'2000-01-01'} ;
DELETE FROM dbo.Test WHERE ID=2;
DELETE FROM dbo.Test WHERE ID=99; --no affect
INSERT INTO dbo.Test(ID,Test1,Test2) VALUES
 (3,'Test3',{d'2001-03-03'})
,(4,'Test4',{d'2001-04-04'})
,(5,'Test5',{d'2001-05-05'});
UPDATE dbo.Test SET Test2=NULL; --all rows
DELETE FROM dbo.Test WHERE ID IN (1,3);
GO

--Check the final status

SELECT * FROM dbo.Test;
SELECT * FROM AuditTest;
GO

--Clean up

DROP TABLE dbo.Test;
GO
DROP TABLE dbo.AuditTest;
GO

SQL Server table audit trigger

Answers (2)

Test.scenario

EDIT: Added support for `INSERT` and `DELETE` and `NULL` values.

Result of second action: UPDATE two rows

Result of `INSERT` action: three new rows

Result of last `DELETE` action

Related Questions

SQL Server table audit trigger

Answers (2)

Test.scenario

EDIT: Added support for INSERT and DELETE and NULL values.

Result of second action: UPDATE two rows

Result of INSERT action: three new rows

Result of last DELETE action

Related Questions

EDIT: Added support for `INSERT` and `DELETE` and `NULL` values.

Result of `INSERT` action: three new rows

Result of last `DELETE` action