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How do you track changes to data in your database? There are a variety of supported auditing methods for SQL Server, including comprehensive C2 security auditing, but what do you do if you're solving a business rather than a security problem, and you're interested in tracking the following kinds of information:

• What data has been updated recently
• Which tables have not been updated recently
• Who modified the price of Steeleye Stout to $20 / unit, and when did they do it?
• What was the unit price for Steeleye Stout before Jon monkeyed with it?

There are a number of ways to design this into your solution from the start, for example:

• The application is designed so that all changes are logged
• All data changes go through a data access layer which logs all changes
• The database is constructed in such a way that logging information is included in each table, perhaps set via a trigger

What if we're not starting from scratch?

But what do you do if you need to add lightweight auditing to an existing solution, in which data can be modified via a variety of direct access methods? When I ran into that challenge, I decided to use Nigel Rivett's SQL Server Auditing triggers. I read about some concern with the performance impact, but this database wasn't forecasted to have a high update rate. Nigel's script works by adding a trigger for INSERT, UPDATE, and DELETE on a single table. The trigger catches data changes, then saves out the information (such as table name, the primary key values, the column name that was altered, and the before and after values for that column) to an Audit table.

I needed to track every table in the database, though, and I expected the database schema to continue to change. I was able to generalize the solution a bit, because the database convention didn't use any no compound primary keys. I created the script listed below, which loops through all tables in the database with the exception of the Audit table, of course, since auditing changes to the audit table is both unnecessary and recursive. I'm also skipping sysdiagrams; you could include any other tables you don't want to track to that list as well.

The nice thing about the script I'm including below is that you can run it after making some schema changes and it will make sure that all newly added tables are included in the change tracking / audit, too.

Here's an example of what you'd see in the audit table for an Update followed by an Insert. Notice that the Update shows type U and a single column updated, while the Insert (type I) shows all columns added, one on each row:



While this information is pretty unstructured, it's not difficult to run some useful reports. For instance, we can easily find things like

• which tables were updated recently
• which tables have not been updated in the past year
• which tables have never been updated
• all changes made by a specific user in a time period
• most active tables in a time period

While it's not as easy, it's possible to backtrack from the current state to determine the state of a row in a table at a certain point in time. It's generally possible to dig out the state of an entire table at a point in time, but a change table isn't a good a fit for temporal data tracking - the right solution there is to start adding Modified By and Modified On columns to the required tables.

Note that we're only tracking data changes here. If you'd like to track schema changes, take a look at SQL Server 2005's DDL triggers.

Enough talking, give us the script!

Sure. I'll repeat that there are some disclaimers to the approach -  performance, it'll only track changes to tables with a primary key, etc. If you want to know more about the trigger itself, I'd recommend starting with Nigel's article. However, it worked great for our project.

Creating a DataTable

To create a DataTable, you need to use System.Data namespace, generally when you create a new class or page, it is included by default by the Visual Studio. Lets write following code to create a DataTable object. Here, I have pased a string as the DataTable name while creating DataTable object.

// instantiate DataTableDataTable dTable = new DataTable("Dynamically_Generated");

Creating Columns in the DataTable

To create column in the DataTable, you need to use DataColumn object. Instantiate the DataColumn object and pass column name and its data type as parameter. Then call add method of DataTable column and pass the DataColumn object as parameter.

// create columns for the DataTable
DataColumn auto = new DataColumn("AutoID", typeof(System.Int32));
dTable.Columns.Add(auto);
// create another column
DataColumn name = new DataColumn("Name", typeof(string));
dTable.Columns.Add(name);
// create one more column
DataColumn address = new DataColumn("Address", typeof(string));
dTable.Columns.Add(address);

Specifying AutoIncrement column in the DataTable

To specify a column as AutoIncrement (naturally it should be an integer type of field only), you need to set some properties of the column like AutoIncrement, AutoIncrementSeed. See the code below, here I am setting the first column "AutoID" as autoincrement field. Whenever a new row will be added its value will automatically increase by 1 as I am specified AutoIncrementSeed value as 1.

// specify it as auto increment field
auto.AutoIncrement = true;
auto.AutoIncrementSeed = 1;
auto.ReadOnly = true;

If you want a particular column to be a unique column ie. you don't want duplicate records into that column, then set its Unique property to true like below.

auto.Unique = true;

Specifying Primary Key column in the DataTable

To set the primary key column in the DataTable, you need to create arrays of column and store column you want as primary key for the DataTable and set its PrimaryKey property to the column arrays. See the code below.

// create primary key on this fieldDataColumn[] pK = new DataColumn[1];
pK[0] = auto;
dTable.PrimaryKey = pK;

Till now we have created the DataTable, now lets populate the DataTable with some data.

Populating data into DataTable

There are two ways to populate DataTable.
Using DataRow object
Look at the code below, I have created a DataRow object above the loop and I am assiging its value to the dTable.NewRow() inside the loop. After specifying columns value, I am adding that row to the DataTable using dTable.Rows.Add method.

// populate the DataTable using DataRow objectDataRow row = null;
for (int i = 0; i < 5; i++)
{
row = dTable.NewRow();
row["AutoID"] = i + 1;
row["Name"] = i + " - Ram";
row["Address"] = "Ram Nagar, India - " + i;
dTable.Rows.Add(row);
}

Instead of using the column name, you can use ColumnIndex too, however it is not suggested as you might want to add a column in the mid of the table then you will need to change your code wherever you have specified the index of the column. Same applies while reading or writing values into Database column.
Asiging the value of column using Arrays
In following code, I have specified the values of every column as the array separated by comma (,) in the Add method of the dTable.Rows.

// manually adding rows using array of valuesdTable.Rows.Add(6, "Manual Data - 1", "Manual Address - 1, USA");
dTable.Rows.Add(7, "Manual Data - 2", "Manual Address - 2, USA");

Modifying data into DataTable

Modifying Row Data
To edit the data of the row, sets its column value using row index or by specifying the column name. In below example, I am updating the 3rd row of the DataTable as I have specified the row index as 2 (dTable.Rows[2]).

// modify certain values into the DataTabledTable.Rows[2]["AutoID"] = 20;
dTable.Rows[2]["Name"] = "Modified";
dTable.Rows[2]["Address"] = "Modified Address";
dTable.AcceptChanges();

Deleting Row
To delete a row into DataTable, call the rows.Delete() method followed by AcceptChanges() method. AcceptChanges() method commits all the changes made by you to the DataTable. Here Row[1] is the index of the row, in this case 2nd row will be deleted as in collection (here rows collection) count start from 0.

// Delete rowdTable.Rows[1].Delete();
dTable.AcceptChanges();

Filtering data from DataTable

To filter records from the DataTable, use Select method and pass necessary filter expression. In below code, the 1st line will simply filter all rows whose AutoID value is greater than 5. The 2nd line of the code filters the DataTable whose AutoID value is greater than 5 after sorting it.

DataRow[] rows = dTable.Select(" AutoID > 5");
DataRow[] rows1 = dTable.Select(" AutoID > 5", "AuotID ASC");

Note that Select method of the DataTable returns the array of rows that matche the filter expression. If you want to loop through all the filtered rows, you can use foreach loop as shown below. In this code, I am adding all the filtered rows into another DataTable.

foreach (DataRow thisRow in rows)
{
// add values into the datatable
dTable1.Rows.Add(thisRow.ItemArray);
}

Working with Aggregate functions (Updated on 18-Nov-08)
We can use almost all aggregate functions with DataTable, however the syntax is bit different than standard SQL.
Suppose we need to get the maximum value of a particular column, we can get it in the following way.

DataRow[] rows22 = dTable.Select("AutoID = max(AutoID)");
string str = "MaxAutoID: " + rows22[0]["AutoID"].ToString();

To get the sum of a particular column, we can use Compute method of the DataTable. Compute method of the DataTable takes two argument. The first argument is the expression to compute and second is the filter to limit the rows that evaluate in the expression. If we don't want any filteration (if we need only the sum of the AutoID column for all rows), we can leave the second parameter as blank ("").

object objSum = dTable.Compute("sum(AutoID)", "AutoID > 7");
string sum = "Sum: " + objSum.ToString();
// To get sum of AutoID for all rows of the DataTable
object objSum = dTable.Compute("sum(AutoID)", "");

Sorting data of DataTable

Oops !. There is no direct way of sorting DataTable rows like filtering (Select method to filter DataRows).
There are two ways you can do this.
Using DataView
See the code below. I have created a DataView object by passing my DataTable as parameter, so my DataView will have all the data of the DataTable. Now, simply call the Sort method of the DataView and pass the sort expression. Your DataView object have sorted records now, You can either directly specify the Source of the Data controls object like GridView, DataList to bind the data or if you need to loop through its data you can use ForEach loop as below.

// Sorting DataTableDataView dataView = new DataView(dTable);
dataView.Sort = " AutoID DESC, Name DESC";
foreach (DataRowView view in dataView)
{
Response.Write(view["Address"].ToString());
}

Using DataTable.Select() method
Yes, you can sort all the rows using Select method too provided you have not specified any filter expression. If you will specify the filter expression, ofcourse your rows will be sorted but filter will also be applied. A small drawback of this way of sorting is that it will return array of DataRows as descibed earlier so if you are planning to bind it to the Data controls like GridView or DataList you will have for form a DataTable by looping through because directly binding arrays of rows to the Data controls will not give desired results.

DataRow[] rows = dTable.Select("", "AutoID DESC");

Writing and Reading XmlSchema of the DataTable

If you need XmlSchema of the DataTabe, you can use WriteXmlSchema to write and ReadXmlSchema to read it. There are several overloads methods of both methods and you can pass filename, stream, TextReader, XmlReader etc. as the parameter. In this code, the schema will be written to the .xml file and will be read from there.

// creating schema definition of the DataTabledTable.WriteXmlSchema(Server.MapPath("~/DataTableSchema.xml"));
// Reading XmlSchema from the xml file we just created
DataTable dTableXmlSchema = new DataTable();
dTableXmlSchema.ReadXmlSchema(Server.MapPath("~/DataTableSchema.xml"));

Reading/Writing from/to Xml

If you have a scenario, where you need to write the data of the DataTable into xml format, you can use WriteXml method of the DataTable. Note that WriteXml method will not work if you will not specify the name of the DataTable object while creating it. Look at the first code block above, I have passed "Dynamically_Generated" string while creating the instance of the DataTable. If you will not specify the name of the DataTable then you will get error as WriteXml method will not be able to serialize the data without it.

// Note: In order to write the DataTable into XML, // you must define the name of the DataTable while creating it
// Also if you are planning to read back this XML into DataTable, you should define the XmlWriteMode.WriteSchema too 
// Otherwise ReadXml method will not understand simple xml file 
dTable.WriteXml(Server.MapPath("~/DataTable.xml"), XmlWriteMode.WriteSchema);
// Loading Data from XML into DataTable
DataTable dTableXml = new DataTable();
dTableXml.ReadXml(Server.MapPath("~/DataTable.xml"));

If you are planning to read the xml you have just created into the DataTable sometime later then you need to specify XmlWriteMode.WriteSchema too as the 2nd parameter while calling WriteXml method of the DataTable otherwise normally WriteXml method doesn't write schema of the DataTable. In the abscence of the schema, you will get error (DataTable does not support schema inference from Xml) while calling ReadXml method of the DataTable.

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