How to Generate XML of Updated Table Columns in SQL Server Efficiently?

Generating XML of Updated Table Columns in SQL Server

SQL Server's COLUMNS_UPDATED function provides insight into modified columns during table updates. However, for scenarios requiring only the updated column values in an XML format, we present a distinct approach.

Automated Routine for Identifying Modified Columns

Rather than manually specifying each column in the SELECT statement, an automated routine can dynamically generate the column list based on table structure. This simplifies trigger development and ensures it adapts to future table modifications. For instance, in a trigger for the DBCustomers table:

CREATE TRIGGER DBCustomers_Insert
ON DBCustomers
AFTER UPDATE
AS
BEGIN
    DECLARE @sql AS NVARCHAR(1024);
    SET @sql = 'SELECT ';

    -- Get the updated columns.
    SELECT TOP (1) COLUMN_NAME
    INTO #ModifiedColumns
    FROM INFORMATION_SCHEMA.COLUMNS_UPDATED()
    ORDER BY COLUMN_NAME;

    WITH RECURSIVE ModifiedColumnsCTE AS (
        SELECT *, ROW_NUMBER() OVER (ORDER BY COLUMN_NAME) AS RowNum, COUNT(*) OVER () AS TotalRows
        FROM #ModifiedColumns
    )
    SELECT COLUMN_NAME INTO #temp
    FROM ModifiedColumnsCTE
    WHERE RowNum < TotalRows
    UNION ALL
    SELECT ',' + COLUMN_NAME
    FROM ModifiedColumnsCTE
    WHERE RowNum = TotalRows;

    -- Append the updated columns to the SELECT statement.
    SET @sql = @sql + STUFF((
        SELECT TOP (1) ColumnName
        FROM #temp
        FOR XML PATH('')
    ), 1, 2, '');

    SET @sql = @sql + ' FROM inserted FOR XML RAW';

    DECLARE @x AS XML;
    SET @x = CAST(EXEC(@sql) AS XML);

    -- Use the @x XML variable for replication purposes.
END

Alternate Solution Using Unpivoting

An alternative approach leverages unpivoting techniques, avoiding COLUMNS_UPDATED altogether:

• Unpivot both inserted and deleted tables, resulting in three columns: unique key, modified column value, and column name.
• Join the unpivoted tables and filter for column value changes, deletions, and insertions.
• Store the results in a separate table for auditing purposes.

This method eliminates the need for dynamic SQL and ensures consistent output formatting, regardless of table structure. It is particularly beneficial for tables with a large number of columns or regular column additions.

Considerations

When dealing with tables having natural primary keys that can change, using an additional column populated with a GUID via the NEWID() function is recommended. This ensures the unpivoting process remains valid for audit trail purposes.

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