Reseeding an Identity Column in a T-SQL Table Variable
Table variables are a great way to store and manipulate data within a T-SQL query. They are similar to temporary tables, but they have some key differences. One of these differences is the ability to have an identity column in a table variable. This can be useful when you need to generate unique values within the table variable. However, what happens when you want to reseed the identity column? In this article, we will explore the process of reseeding an identity column in a T-SQL table variable.
First, let's briefly review what an identity column is. An identity column is a column that automatically generates a unique numeric value for each row in a table. This is useful for creating primary keys or other unique identifiers. In a traditional table, the identity column can be reseeded by using the DBCC CHECKIDENT command. However, this command does not work on table variables.
So, how can we reseed an identity column in a T-SQL table variable? The answer is to use the ALTER TABLE statement. This statement allows us to alter the structure of a table variable, including the identity column. Let's take a look at an example.
Imagine we have a table variable called @Employees with three columns: ID (identity column), Name, and Department. We have inserted some data into this table variable, and the ID column has been automatically populated with unique values. Now, we want to reseed the identity column to start from 1001 instead of 1. We can achieve this by using the following code:
```
DECLARE @Employees TABLE (
ID INT IDENTITY(1,1),
Name VARCHAR(50),
Department VARCHAR(50)
)
INSERT INTO @Employees (Name, Department)
VALUES ('John Smith', 'Sales'), ('Jane Doe', 'Marketing'), ('Bob Johnson', 'Finance')
ALTER TABLE @Employees ALTER COLUMN ID IDENTITY(1001,1)
```
In this code, we first declare our table variable @Employees with the ID column set as an identity column. Then, we insert some sample data into the table variable. Finally, we use the ALTER TABLE statement to alter the ID column and specify a new seed and increment value. Now, if we insert any additional rows into the table variable, the ID column will start from 1001 and increment by 1 for each new row.
It's important to note that when we alter the identity column in a table variable, all existing rows will be renumbered based on the new seed and increment values. This means that the previous IDs will no longer be unique and should not be relied upon for any relationships or joins.
In addition to reseeding the identity column, the ALTER TABLE statement also allows us to change the data type or size of a column, add or drop columns, and set constraints. This makes it a powerful tool for modifying the structure of a table variable.
In conclusion, reseeding an identity column in a T-SQL table variable can be easily done using the ALTER TABLE statement. This allows us to change the starting value and increment of the identity column, making it more flexible for our needs. Just remember to be cautious when altering the structure of a table variable, as it can affect the existing data within it.
