Inserting value for identity column in a temporary table in sql

Temporary tables in SQL are widely used for storing intermediate results during query execution, especially in complex procedures or batch operations. In certain scenarios, we may need to insert values into identity columns of temporary tables. By default, identity columns auto-generate their values.

In this article, We will guide go through the concept of identity columns in temporary tables, practical scenarios and how to insert custom values into them in detail.

What Are Identity Columns in SQL?

An identity column is a special column type in SQL that auto-generates unique integer values for each row. It is commonly utilized as a primary key to uniquely identify each record in a table.

Syntax to Create an Identity Column:

CREATE TABLE table_name (    column_name INT IDENTITY(start_value, increment_value),    other_columns ...);

• start_value: The initial value for the identity column.
• increment_value: The step by which the value increases for each new row.

Temporary Tables in SQL

Temporary tables are database tables that exist temporarily during a database session or transaction. They are ideal for intermediate calculations or storing temporary data. Temporary tables can also include identity columns.

Syntax of Creating a Temporary Table with an Identity Column:

CREATE TABLE #TempTable (    ID INT IDENTITY(1, 1),    Name VARCHAR(50));

Explanation: the ID column is an identity column that starts at 1 and increments by 1.

Can We Insert Values into Identity Columns?

By default, SQL does not allow explicit insertion into identity columns to maintain their uniqueness and auto-increment properties. However, SQL provides a way to override this restriction using the SET IDENTITY_INSERT command.

To insert explicit values into an identity column, we need to enable IDENTITY_INSERT for the table. This feature is commonly used when migrating data or performing certain maintenance tasks.

Example of Inserting Values into an Identity Column of a Temporary Table

Lets perform an example in stepwuise manner as below

Step 1: Create a Temporary Table

Lets write a query to create temporary table called TempEmployees to perform further operations as below:

CREATE TABLE #TempEmployees (
    EmpID INT IDENTITY(1001, 1),
    Name VARCHAR(50)
);

Explanation: The CREATE TABLE statement defines a temporary table #TempEmployees with:

• EmpID as an identity column (auto-generated values starting at 1001, incrementing by 1).
• Name as a VARCHAR(50) column to store employee names (up to 50 characters)

Step 2: Insert Default Values

By default, we can insert rows without specifying values for the identity column:

INSERT INTO #TempEmployees (Name)
VALUES 
    ('Alice'),
    ('Bob'),
    ('Charlie');

Output:

Step 3: Enable IDENTITY_INSERT

Lets insert specific values into the EmpID identity column of the temporary table #TempEmployees by enabling IDENTITY_INSERT. Add rows with explicitly defined EmpID and Name values.

SET IDENTITY_INSERT #TempEmployees ON;

INSERT INTO #TempEmployees (EmpID, Name)
VALUES 
    (2001, 'David'),
    (2002, 'Eve');

Output:

Step 4: Disable IDENTITY_INSERT

After inserting the values, disable IDENTITY_INSERT to restore the default behavior:

SET IDENTITY_INSERT #TempEmployees OFF;

Explanation:

• The SET IDENTITY_INSERT statement allows explicit insertion of values into the identity column (EmpID) of a table. This feature is typically disabled by default to prevent manual value entry into identity columns. SET IDENTITY_INSERT #TempEmployees ON: Enables manual insertion into the EmpID column.
  
• The INSERT INTO statement specifies values for both EmpID and Name. After the operation, it's essential to turn off IDENTITY_INSERT using SET IDENTITY_INSERT #TempEmployees OFF to restore default behavior.

Conclusion

Overall, Inserting explicit values into an identity column in temporary tables is a powerful feature for advanced SQL workflows. By using the IDENTITY_INSERT command, you can override default identity behaviors to meet specific requirements, such as data migration, testing or debugging. Always remember to disable IDENTITY_INSERT after the operation to maintain the integrity of your database.

With these techniques, you can handle identity columns effectively in temporary tables, ensuring both flexibility and reliability in your SQL operations.