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How to retrieve the automatic @@IDENTITY value from an insert to a SQL Server database in Visual C++


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This article was previously published under Q185821
Note Microsoft Visual C++ .NET (2002) and Microsoft Visual C++ .NET (2003) support both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.

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Summary

When you insert values into a table with an identity column, SQL Server automatically generates the next identifier based on the last used identity value. You might want to retrieve this automatically generated identity value immediately after an insert.

The Ident.exe sample demonstrates how to retrieve the automatic @@IDENTITY value from an insert into a Microsoft SQL Server database. The sample consists of a simple MFC AppWizard project based on the CRecordset class. The project was modified slightly to allow inserts into the table and to retrieve the automatically generated identity value for the insert. It uses the pubs data source name (DSN), although any DSN will work, and a table called tblIdentity.

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More information


The following files are available for download from the Microsoft Download Center:
For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base:
119591� How to Obtain Microsoft Support Files from Online Services
Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help to prevent any unauthorized changes to the file.

Ident.exe includes an algorithm and sample code that illustrate how to retrieve the identity value from a SQL Server database. You can use this method for multiple processes or threads doing simultaneous inserts into the same table. (Note that threads cannot share connections.)

Project description

A master identity table, called tblIdentity, is created. It consists of the identity, table name, and server process ID number (SPID). An insert trigger into tblIdentity is created for each table in the database that satisfies the following criteria:

  • Inserts are performed on the table.
  • The table has an identity column with values that are automatically generated by SQL Server.
  • The automatically generated identity value is needed immediately after the insertion.
The following SQL script, located in the Master.sql file, generates the sample table, insertTbl:
   CREATE TABLE insertTbl (myAuto int IDENTITY (1, 1) NOT NULL ,
                           lname varchar (15) NOT NULL ,
                           fname varchar (15) NOT NULL )
				
Because names are not unique, the code uses the identity property of SQL Server to automatically generate a unique key.

The following SQL script, located in the Master.sql file, creates an insert trigger for insertTbl:
   CREATE TRIGGER trgAudit ON insertTbl FOR INSERT
      AS
      INSERT INTO tblIdentity VALUES (@@IDENTITY, 'insertTbl', @@SPID)
				
Immediately after an insertion into tblIdentity, the trigger fires and stores:

  • the identity SQL Server generated (@@IDENTITY),
  • the table name ("insertTbl" in this case), and
  • the SPID (the server process ID number of the current process, guaranteed to be unique for each connection).
After insertion, the program calls a stored procedure (called sp_getID) to retrieve the identity value.

The code calls the stored procedure sp_getID (with a negative value for the spid parameter) in the one time initialization to get the SPID (the server process ID number of the current process, guaranteed to be unique for each connection). Applications with multiple threads inserting into the same table must not share a connection. Each thread must have it's own connection to guarantee a unique SPID.

NOTE: Applications with multiple threads sharing a connection must use a more complicated algorithm that includes the thread ID and the SPID to establish a unique insertion.

After each insertion, the program calls sp_getID and passes the table name where the insert occurred and the SPID that was retrieved in the initial call to sp_getID. The stored procedure sp_getID returns the unique identity value that SQL Server generated on the insert.

Run the sample

To use the sample application, run the Master.sql file in the SQL Query Tool from SQL Enterprise Manager. After running the script, you can verify that the tables and triggers were created successfully by running the following SQL statement:
   insert into insertTbl (lname,fname) VALUES ('Smith','Joe')
   select * from tblIdentity
				
The Results tab should look similar to the following:
   iID         strTable        SPID
   ----------- --------------- -----------
   5           insertTbl       11

   (1 row(s) affected)
				
The iID and SPID values you see will probably not be 5 and 11. The tblIdentity values verify that the Master.sql script was successful.

Note The sample code uses a sub-optimal method to display the identity value and is used for illustration purposes only.

The sample uses the SQLExecDirect function to run the stored procedure and retrieve the identity value.

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References

For more information, click the following article number to view the article in the Microsoft Knowledge Base:
163446� PRB: Guarantee @@IDENTITY Value on a Per Table Basis

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Keywords: kbhowto, kbdownload, kbsqlprog, kbdatabase, kbfile, kbsample, kbdriver, atdownload, KB185821

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Article Info
Article ID : 185821
Revision : 5
Created on : 9/1/2005
Published on : 9/1/2005
Exists online : False
Views : 280