C Generate Incremental Key If Not Supplied

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By: Ben Richardson Updated: 2018-09-26 Comments (6) Related: More >Identities
  • Theoretically, this should always create a unique key. However, for some reason DateTime.Now.Ticks is not unique. For example, 5 records in a row might all have the key 64435908, and the next 7 records might have the key 64592164, even though I am generating.
  • Oct 23, 2008  Hi all, The other day a colleague of mine asked me if I had a.NET version of the C sample in How to generate key pairs, encrypt and decrypt data with CryptoAPI post. C sample calls CryptoAPI directly (and you know we can do the same thing in.NET through P/Invoke), but the idea was to use System.Security classes in order to get a pure.NET solution.


While designing a SQL Server database, the primary key column is often set to auto-increment. To do this, the IDENTITY constraint is set on the primary key column. The starting position and the increment step are passed as parameters to the IDENTITY column. Then whenever a new record is inserted, the value of the IDENTITY column is incremented by the pre-defined step, usually a number. Now if a record is deleted, the IDENTITY column value for that record is also deleted. If a new record is inserted, its value for the IDENTITY column will be incremented from the previous figure in the column. It is not possible to reuse the value that was originally used by the now deleted record. If you try to specify the value for the IDENTITY column, an error will be thrown. So how do you reuse the value that was assigned to the deleted record?


Dismiss Join GitHub today. GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. Create table t1 (id integer not null, field1 varchar(20) not null); To make column ID autoincrement, we don't have to do anything special to the column itself, but we need to create a BEFORE INSERT trigger that will put a new value each time a new record is inserted. To make sure we get unique values consistently we use a GENERATOR (a.k.a. Az storage blob generate-sas: Generate a shared access signature for the blob. Az storage blob incremental-copy: Manage blob incremental copy operations. Az storage blob incremental-copy cancel: Aborts a pending copyblob operation, and leaves a destination blob with zero length and full metadata. Az storage blob incremental-copy start.

The solution to this problem is to switch on the SET IDENTITY INSERT flag, which is off by default. Switching the SET IDENTITY INSERT flag to ON allows for the insertion of any random value to the IDENTITY column, as long as it doesn't already exist.

In this article, I will explain (with the help of an example) how to insert a missing value into the IDENTITY column.

Setup Example Table and Data

First let’s create some dummy data. We will execute our sample queries on this new database.

In the script, we create a dummy database “School”. Next, we execute the script that creates a table named “Students”. If you look at the table design, you can see that it contains three columns Id, StudentName and StudentAge. The Id column is the primary key column with an IDENTITY constraint. Both the seed and step values for IDENTITY are set to 2. This means that the first record in the “Students” database will have the Id value of 2 and each subsequent record will have a value incremented by 2. Finally, we insert 5 random records into the Students table.

Now if you select all the records from the Students table, you will see that Id column will contain a sequence of values starting at 2 and incremented by 2 in each row. Execute the following script:

The output looks like this:

You can see that the first record has an id value of 2 while the 5th has a value of 10.

Now, suppose one of the students leaves the school and we want to delete his record. We can do so using a simple DELETE statement. Let’s delete the record of Jon:

Now if you again look at all the records in the Students table using SELECT statement, you will see the following output: Mac os x boot camp external drive windows 10.

You can see from the output that record of the student “Jon” with Id 6 has been deleted.

Now let’s try to insert a record of a new student and see what Id it gets:

The above script inserts a record of a new student named “Jessica”, aged 27, to the Students table.

To see the Id assigned to the Jessica, again retrieve all records from the Students table using the SELECT statement as shown below:

The output looks like this:

You can see from the output that Jessica has been assigned the Id 12 instead of the Id 6 vacated by Jon. The reason for this behavior is the fact that by default IDENTITY column assigns a value to a new record by adding the step to previous maximum value in the column instead of filling the vacant values in the column. Since the previous maximum value in the Id column was 10, therefore Jessica is assigned 12, since the step is 2.

Depending upon the business rules of the application being developed, this behavior can be correct. For instance, a School may have a rule that even if a student leaves the school, his/her Id cannot be assigned to a new student. On the other hand, there can be a school that reassigns the Id of a student who leaves the school, to a new student.

Manually Insert Record with specific ID value

In the latter case, one of the solutions is to manually insert the Id value for the new student.

C Generate Incremental Key If Not Supplied Mean

Let’s try to add a record of a new student and manually set the value for the Id column to 6 as shown below:

The above script inserts a record of a new student named “Nick”, aged 22, and Id 6, to the students table. When you try to execute the above script, an error will be thrown which looks likes this:

Msg 8101, Level 16, State 1, Line 26
An explicit value for the identity column in table 'Students' can only be specified when a column list is used and IDENTITY_INSERT is ON.

In simple words, the error says that since the flag IDENTITY_INSERT is off for the Id column, we cannot manually insert any values. Another important consideration is that we need to specify the column names as well while inserting data to IDENTITY column.

If we try to just specify the column names as mentioned in the above error message as follows:

We get this error message.

Msg 544, Level 16, State 1, Line 35
Cannot insert explicit value for identity column in table 'Students' when IDENTITY_INSERT is set to OFF.

To manually insert a new value into the Id column, we first must set the IDENTITY_INSERT flag ON as follows:

To set the IDENTIT_INSERT flag ON we need to use the SET statement followed by the flag name and the name of the table.

Now if we again try to insert the record of the student “Nick” with Id 6, no error will be thrown. Execute the following statement again:

You can see that we have specified the name of the columns as well for inserting a record.

Now again use SELECT statement to retrieve all records from students table in order to view if our new record has been inserted or not. The SELECT statement will return the following records.

You can see from the output that record of a new student named “Nick”, aged 22, and Id 6 has been inserted to the Students table.

Try Inserting Duplicate Values

If we try to insert duplicate values as follows:

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We will get this error, since ID is the primary key for the table which has to be a unique value.

Msg 2627, Level 14, State 1, Line 35
Violation of PRIMARY KEY constraint 'PK__Students__3214EC071AD8EC3F'. Cannot insert duplicate key in object 'dbo.Students'. The duplicate key value is (6).
The statement has been terminated.

Note, that if the ID column was not a Primary Key we would be able to insert duplicate records.


When you SET INDENTITY_INSERT ON it will stay on for the entire session (the time the query window is open). So once this is set you can insert as many records as you want. Also, this only applies for the session where this is turned on, so if you open another query window you would need to set this ON for that query window.

To turn off this option for the session, you would issue the following statement.

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Next Steps
  • In this article, we saw that how we can use the SET IDENTITY_INSERT flag as ON in order to insert a record in the IDENTITY column which is not possible with default settings.
  • Check out these related articles:

Last Updated: 2018-09-26

About the author
Ben is the owner of Acuity Training, a UK based IT training business offering SQL training up to advanced administration courses.
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Creating and managing keys is an important part of the cryptographic process. Symmetric algorithms require the creation of a key and an initialization vector (IV). The key must be kept secret from anyone who should not decrypt your data. The IV does not have to be secret, but should be changed for each session. Asymmetric algorithms require the creation of a public key and a private key. The public key can be made public to anyone, while the private key must known only by the party who will decrypt the data encrypted with the public key. This section describes how to generate and manage keys for both symmetric and asymmetric algorithms.

Symmetric Keys

C Generate Incremental Key If Not Supplied Name

The symmetric encryption classes supplied by the .NET Framework require a key and a new initialization vector (IV) to encrypt and decrypt data. Whenever you create a new instance of one of the managed symmetric cryptographic classes using the parameterless constructor, a new key and IV are automatically created. Anyone that you allow to decrypt your data must possess the same key and IV and use the same algorithm. Generally, a new key and IV should be created for every session, and neither the key nor IV should be stored for use in a later session.

To communicate a symmetric key and IV to a remote party, you would usually encrypt the symmetric key by using asymmetric encryption. Sending the key across an insecure network without encrypting it is unsafe, because anyone who intercepts the key and IV can then decrypt your data. For more information about exchanging data by using encryption, see Creating a Cryptographic Scheme.

The following example shows the creation of a new instance of the TripleDESCryptoServiceProvider class that implements the TripleDES algorithm.

When the previous code is executed, a new key and IV are generated and placed in the Key and IV properties, respectively.

Sometimes you might need to generate multiple keys. In this situation, you can create a new instance of a class that implements a symmetric algorithm and then create a new key and IV by calling the GenerateKey and GenerateIV methods. The following code example illustrates how to create new keys and IVs after a new instance of the symmetric cryptographic class has been made.

When the previous code is executed, a key and IV are generated when the new instance of TripleDESCryptoServiceProvider is made. Another key and IV are created when the GenerateKey and GenerateIV methods are called.

Asymmetric Keys

The .NET Framework provides the RSACryptoServiceProvider and DSACryptoServiceProvider classes for asymmetric encryption. These classes create a public/private key pair when you use the parameterless constructor to create a new instance. Asymmetric keys can be either stored for use in multiple sessions or generated for one session only. While the public key can be made generally available, the private key should be closely guarded.

A public/private key pair is generated whenever a new instance of an asymmetric algorithm class is created. After a new instance of the class is created, the key information can be extracted using one of two methods:

  • The ToXmlString method, which returns an XML representation of the key information.

  • The ExportParameters method, which returns an RSAParameters structure that holds the key information.

Both methods accept a Boolean value that indicates whether to return only the public key information or to return both the public-key and the private-key information. An RSACryptoServiceProvider class can be initialized to the value of an RSAParameters structure by using the ImportParameters method.

Asymmetric private keys should never be stored verbatim or in plain text on the local computer. If you need to store a private key, you should use a key container. For more on how to store a private key in a key container, see How to: Store Asymmetric Keys in a Key Container.

The following code example creates a new instance of the RSACryptoServiceProvider class, creating a public/private key pair, and saves the public key information to an RSAParameters structure.

C Generate Incremental Key If Not Supplied Download

See also

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