Understanding Identity Columns: How to Track Changes with Output Clause in SQL Server

Understanding the Problem and Solution

The problem presented is a classic example of how to track changes or insertions in SQL Server, particularly when dealing with identity columns. The question asks how to perform an “insert after insert” operation, where the result ID’s from the first insert are used to combine data.

Background and Context

To approach this problem, we need to understand a few key concepts:

Identity Columns: These are columns that automatically generate unique IDs when data is inserted. SQL Server uses these IDs to create primary keys.
OUTPUT Clause: This clause is used in conjunction with the INSERT statement to return the ID or other values generated during the insert operation.
Auditing and Tracking Changes: The solution presented involves creating an audit table to track changes made to data.

Step-by-Step Solution

Part 1: Creating a Temporary Table for Auditing

CREATE TABLE #AUDIT ( EntityKey int not null default -1, OldMyName varchar(128) null, NewMyName varchar(128) null , Tag varchar(64)  );

This step creates a temporary table called #AUDIT where we will store the audit information.

Part 2: Creating a Temporary Table to Mimic the Real Table

CREATE TABLE #MEMBERSREALTABLEMIMIC (MyIdentity INT IDENTITY (3001, 3) , MyName varchar(128) );

This step creates another temporary table called #MEMBERSREALTABLEMIMIC that will be used as a placeholder for the real Member table.

Part 3: First Insertion

DECLARE @tmpTable TABLE (UserId INT, Name nvarchar(50), Department nvarchar(50))
DECLARE @xml XML=
N'&lt;user&gt;&lt;userId&gt;1&lt;/userId&gt;&lt;name&gt;John&lt;/name&gt;&lt;department&gt;A&lt;/department&gt;&lt;/user&gt;
&lt;user&gt;&lt;userId&gt;2&lt;/userId&gt;&lt;name&gt;Jane&lt;/name&gt;&lt;department&gt;B&lt;/department&gt;&lt;/user&gt;';
INSERT INTO @tmpTable
SELECT 
    a.b.value('(./userId)[1]', 'int') AS UserId,
    a.b.value('(./name)[1]', 'nvarchar(50)') AS Name,
    a.b.value('(./department)[1]', 'nvarchar(50)') AS Department
FROM @xml.nodes('/user') a(b)

This step performs the first insertion into the @tmpTable using an XML string as input.

Part 4: Tracking Changes with the #AUDIT Table

INSERT INTO #MEMBERSREALTABLEMIMIC (MyName)
OUTPUT inserted.MyIdentity , null , inserted.MyName , 'Employee Inserted' into #AUDIT ( EntityKey , OldMyName , NewMyName , Tag)
SELECT Name FROM @tmpTable;

This step inserts the data from @tmpTable into #MEMBERSREALTABLEMIMIC, and uses the OUTPUT clause to track changes made during this operation, which is then inserted into the #AUDIT table.

Part 5: Combining Data with Result IDs

SELECT * from #MEMBERSREALTABLEMIMIC
SELECT * from #AUDIT

This step retrieves the data from #MEMBERSREALTABLEMIMIC and #AUDIT, which now contains the result IDs generated during the first insertion.

Part 6: Dropping Temporary Tables

DROP TABLE #MEMBERSREALTABLEMIMIC
DROP TABLE #AUDIT;

This step drops both temporary tables to clean up after the experiment.

Conclusion

The provided solution demonstrates how to track changes and insertions in SQL Server, using a combination of identity columns, the OUTPUT clause, and an audit table. By understanding these concepts and applying them correctly, developers can efficiently track changes made during data insertion operations, ensuring that their database remains consistent and reliable.

Last modified on 2023-10-05