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Quick SQL Server Auditing Setup

, 16 Aug 2012
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Setting up a lightweight audit function for a SQL Server db.

Introduction

There is an obvious need to implement an audit function in database applications. Having devised this solution I could not see a similar approach taken by anyone else, hence I am presenting here.

My audit tool implements the following features:

  • an Audit table storing details of all insert, update and delete operations
  • only data changes are captured, i.e. tool does not capture the details of the entire row on every data change
  • data change operations are grouped in the Audit table, e.g. if 10 rows are updated each Audit row will have an identical Operation Ref
  • an Audit Config table allows control over which tables and columns to apply the audit tool.

Using the code

The audit tool is implemented like so:

Any table implementing the audit tool will have three triggers. Each trigger will:

  1. store the inserted and deleted tables to a local temporary table;
  2. get some dynamic SQL from a GetAuditSQL function; 
  3. execute the SQL code (this generates the Audit data); 
  4. delete the temporary tables. 
The trigger SQL code is below:
ALTER TRIGGER [dbo].[Customer_DELETE_Audit] ON [dbo].[Customer] AFTER DELETE
AS
BEGIN
SET NOCOUNT ON
select * into #tmpDeleted from deleted;
declare @sql nvarchar(max);
select @sql = [audit].[GetAuditSQL]('dbo','Customer','delete','','#tmpDeleted');
if isnull(@sql,'') <> '' exec sp_executesql @sql;
drop table #tmpDeleted;
END
GO
 

ALTER TRIGGER [dbo].[Customer_INSERT_Audit] ON [dbo].[Customer] AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
select * into #tmpInserted from inserted;
declare @sql nvarchar(max);
select @sql = [audit].[GetAuditSQL]('dbo','Customer','insert','#tmpInserted','');
if isnull(@sql,'') <> '' exec sp_executesql @sql;
drop table #tmpInserted;
END
GO
 
ALTER TRIGGER [dbo].[Customer_UPDATE_Audit] ON [dbo].[Customer] AFTER UPDATE AS
BEGIN
SET NOCOUNT ON
select * into #tmpInserted from inserted;
select * into #tmpDeleted from deleted;
declare @sql nvarchar(max);
select @sql = [audit].[GetAuditSQL]('dbo','Customer','update','#tmpInserted','#tmpDeleted');
if isnull(@sql,'') <> '' exec sp_executesql @sql;
drop table #tmpInserted;
drop table #tmpDeleted;
END
GO

The GetAuditSQL code will iterate over the columns for the given table in AuditConfig and generate code that:

  1. compares the deleted and inserted data (from the trigger) to see where changes exist; 
  2. derive old and new values from the deleted and inserted data;
  3. insert rows into the Audit table. 

The GetAuditSQL code is below:

ALTER function [audit].[GetAuditSQL](
@Schemaname varchar(100),
@Tablename varchar(100),
@OperationType varchar(100),
@InsertedTableName varchar(100),
@DeletedTableName varchar(100)) returns nvarchar(max)
as
begin
    declare @OpRef varchar(100) = isnull((select max(OperationRef)+1 FROM audit),1)
    declare @retval nvarchar(max) = N''
    if not exists (select 0 from AuditConfig where Tablename = @Tablename and EnableAudit = 1)
        return @retval
    set @retval = N'
            insert into [audit].[Audit] ([Login],OperationRef,Schemaname,
            Tablename,OperationType,ColumnName,RowID,OldValue,NewValue) 
            select SYSTEM_USER, ''' + @OpRef + ''',''' + @Schemaname + ''',''' + 
            @Tablename + ''',''' + @OperationType+ ''', * from 
            ('
    declare @AddUnion bit = 0;
    declare @ColumnName varchar(100) = ''
    declare ac cursor for select ColumnName from AuditConfig 
            where Tablename = @Tablename and EnableAudit = 1
    open ac
    fetch next from ac into @ColumnName
    while @@FETCH_STATUS = 0 
    begin
        if @AddUnion = 1 SET @retval = @retval + N' union '
        if @OperationType = 'insert'
        begin
            SET @retval = @retval + N'
                select ''' + @ColumnName + ''' ColName, i.ID, NULL OldValue, 
                convert(nvarchar(max),i.['+ @ColumnName + 
                ']) NewValue from [' + @InsertedTableName + '] i'
        end
        if @OperationType = 'update'
        begin
            SET @retval = @retval + N'
                select ''' + @ColumnName + ''' ColName, i.ID, convert(nvarchar(max),d.['+ @ColumnName + ']) OldValue, 
                   convert(nvarchar(max),i.['+ @ColumnName + ']) NewValue from [' + @InsertedTableName + '] i JOIN [' + 
                   @DeletedTableName + '] d ON i.ID = d.ID AND (i.['+ @ColumnName + 
                   '] <> d.['+ @ColumnName + '] OR i.['+ @ColumnName + 
                   '] IS NOT NULL AND d.['+ @ColumnName + '] IS NULL)'
        end
        if @OperationType = 'delete'
        begin
            SET @retval = @retval + N'
                select ''' + @ColumnName + ''' ColName, i.ID, 
                convert(nvarchar(max),i.['+ @ColumnName + ']) OldValue, 
                NULL NewValue from [' + @DeletedTableName + '] i'
        end
        SET @AddUnion = 1
        fetch next from ac into @ColumnName
    end
    close ac
    deallocate ac
    SET @retval = @retval + N') d'
    return @retval
end  

The script file attached to this article will set up the audit tool in it's entirety:

  1. Set up the Audit and AuditConfig tables
  2. Set up the GetAuditSQL function 
  3. Set up two SPs (these can be called when new tables or columns are introduced to the database):
    1. to auto-populate the AuditConfig table with all table and column names; 
    2. set up all triggers.
  4. Execute the two SPs, applied to a specific schema (dbo by default) 
There are notes in the header that should be read before executing the script. Hope this is helpful!

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Andy Taw
Software Developer
United Kingdom United Kingdom
No Biography provided

Comments and Discussions

 
QuestionSome errors [modified] PinmemberKanwal Shehzad29-Aug-12 21:34 
AnswerRe: Some errors PinmemberHitendra Jadav28-Feb-13 8:44 

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