Question

For entities in my application, I am planning to log common meta-data like DateCreated, DateModified, IPAddress, etc. Does it make sense to add these columns in entity tables or is it better to have a single table for logging meta-data where each row has link back to the item that it corresponds to? Later for purpose of reporting and analysis, I can create desired views.

Answer 1

I use a special query that adds all these common columns (I call them audit columns) to all tables (using a cursor going over the information schema), which makes it easy to apply them to new databases.

My columns are (SQL Server specific):

RowId UNIQUEIDENTIFIER NOT NULL DEFAULT (NEWID()),
Created DATETIME NOT NULL DEFAULT (GETDATE()),
Creator NVARCHAR(256) NOT NULL DEFAULT(SUSER_SNAME())
RowStamp TIMESTAMP NOT NULL

Now, in a fully normalised schema, I’d only need RowId, which would link to an Audit table containing the other rows. In fact, on reflection, I almost wish I had gone down this route – mainly because it makes the tables ugly (in fact I leave these columns out of database schema diagrams).

However, when dealing with very large data sets, you do get a performance boost from having this data within the table, and I haven’t experienced any problems with this system to date.

Edit: Might as well post the code to add the audit columns:

DECLARE AuditCursor CURSOR FOR
  SELECT TABLE_SCHEMA, TABLE_NAME from INFORMATION_SCHEMA.TABLES
  WHERE TABLE_TYPE = 'BASE TABLE'
  AND OBJECTPROPERTY(OBJECT_ID(QUOTENAME(TABLE_SCHEMA) + '.' + QUOTENAME(TABLE_NAME)), 'IsMSShipped') = 0
  AND TABLE_NAME NOT IN ('sysdiagrams')
  AND TABLE_NAME NOT LIKE 'dt_%'

  -- NB: you could change the above to only do it for certain tables

OPEN AuditCursor
  DECLARE @schema varchar(128), @table varchar(128)

  FETCH NEXT FROM AuditCursor INTO @schema, @table

  WHILE @@FETCH_STATUS  -1
  BEGIN

    PRINT '* Adding audit columns to [' + @schema + '].[' + @table + ']...'

    IF NOT EXISTS  (SELECT NULL
        FROM information_schema.columns
        WHERE table_schema = @schema
        AND table_name = @table
        AND column_name = 'Created')
    BEGIN
      DECLARE @sql_created varchar(max)
      SELECT  @sql_created = 'ALTER TABLE [' + @schema + '].[' + @table + '] ADD [Created] DATETIME NOT NULL CONSTRAINT [DF_' + @table + '_Created] DEFAULT (GETDATE())'

      EXEC  ( @sql_created )
      PRINT ' - Added Created'
    END
    ELSE
      PRINT ' - Created already exists, skipping'

    IF NOT EXISTS  (SELECT NULL
        FROM information_schema.columns
        WHERE table_schema = @schema
        AND table_name = @table
        AND column_name = 'Creator')
    BEGIN
      DECLARE @sql_creator varchar(max)
      SELECT  @sql_creator = 'ALTER TABLE [' + @schema + '].[' + @table + '] ADD [Creator] VARCHAR(256) NOT NULL CONSTRAINT [DF_' + @table + '_Creator] DEFAULT (SUSER_SNAME())'

      EXEC  ( @sql_creator )
      PRINT ' - Added Creator'
    END
    ELSE
      PRINT ' - Creator already exists, skipping'

    IF NOT EXISTS  (SELECT NULL
        FROM information_schema.columns
        WHERE table_schema = @schema
        AND table_name = @table
        AND column_name = 'RowId')
    BEGIN
      DECLARE @sql_rowid varchar(max)
      SELECT  @sql_rowid = 'ALTER TABLE [' + @schema + '].[' + @table + '] ADD [RowId] UNIQUEIDENTIFIER NOT NULL CONSTRAINT [DF_' + @table + '_RowId] DEFAULT (NEWID())'

      EXEC  ( @sql_rowid )
      PRINT ' - Added RowId'
    END
    ELSE
      PRINT ' - RowId already exists, skipping'

    IF NOT EXISTS  (SELECT NULL
        FROM information_schema.columns
        WHERE table_schema = @schema
        AND table_name = @table
        AND (column_name = 'RowStamp' OR data_type = 'timestamp'))
    BEGIN
      DECLARE @sql_rowstamp varchar(max)
      SELECT  @sql_rowstamp = 'ALTER TABLE [' + @schema + '].[' + @table + '] ADD [RowStamp] ROWVERSION NOT NULL'
      EXEC  ( @sql_rowstamp )
      PRINT ' - Added RowStamp'
    END
    ELSE
      PRINT ' - RowStamp or another timestamp already exists, skipping'

    -- basic tamper protection against non-SA users
    PRINT ' - setting DENY permission on audit columns'
    DECLARE @sql_deny VARCHAR(1000)
    SELECT  @sql_deny = 'DENY UPDATE ON [' + @schema + '].[' + @table + '] ([Created]) TO [public]'
      + 'DENY UPDATE ON [' + @schema + '].[' + @table + '] ([RowId]) TO [public]'
      + 'DENY UPDATE ON [' + @schema + '].[' + @table + '] ([Creator]) TO [public]'

    EXEC (@sql_deny)

    PRINT '* Completed processing [' + @schema + '].[' + @table + ']'
    FETCH NEXT FROM AuditCursor INTO @schema, @table

  END

CLOSE AuditCursor
DEALLOCATE AuditCursor
GO