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How to Change SQL Server Columns from NULL to Not NULL

Navigating SQL Server: Altering Column Nullability

In the realm of database management, few tasks are as critical and potentially tricky as altering the nullability of columns. The operation to alter column null not null in SQL Server is not merely a change in data structure—it can significantly affect how your database behaves. This essay will guide you through the process, discuss the limitations of not null columns, and provide best practices for altering SQL Server columns.

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Understanding NULL Values in SQL Server

Before diving into the process of changing data structure of a column, it's crucial to understand what a NULL value represents in SQL Server. A NULL value is essentially a placeholder for an unknown or inapplicable value. A column allows NULL values by default, but you may want to set a column to not null in SQL Server to ensure every row in your table has a value for that column.

Altering Column from Null to Not Null: Syntax and Pitfalls

The SQL Server ALTER TABLE syntax is used to modify the properties of an existing table. To change a column from null to not null, you would use the ALTER COLUMN clause. Here is a simple example:

ALTER TABLE table_name
ALTER COLUMN column_name column_type NOT NULL

However, there's a potential pitfall here. If you attempt to change a column with existing NULL values to NOT NULL, SQL Server will throw an error (SQL Server error 23000). The reason is simple: SQL Server cannot guarantee that every row in the altered column will have a value, which is a requirement for NOT NULL columns.

Unnullifying Existing Column Data

To avoid the aforementioned error, it's necessary to "unnullify" existing column data before altering the column to not null. Essentially, this means replacing all NULL values with a non-NULL value. This is done using the SQL Server UPDATE command:

UPDATE table_name
SET column_name = replacement_value
WHERE column_name IS NULL

Once all NULL values have been replaced, you can then safely proceed with the ALTER TABLE command to set the column to not null. Remember to verify altered nullability by using the SQL select column with Null values command:

SELECT column_name
FROM table_name
WHERE column_name IS NULL

If the column has been successfully altered to NOT NULL, this query should return an empty set.

Altering Existing SQL Server Data: Safety Precautions and Best Practices

One significant safety precaution when altering existing SQL Server data is to always back up your data before making changes. This is especially true when altering column data structures or altering a column from null to not null.

Additionally, try to avoid altering columns during peak database usage times. Changes to a table structure can lock the table and disrupt normal database operations.

Real-World Scenario: SQL Server Phone Column

To illustrate this process, let's consider a real-world example using an SQL Server phone column. Suppose we have a table of customer data, and we want to ensure that every customer has a phone number listed.

First, we need to insert data into SQL Server table for any customers who don't have a phone number listed. Let's say we'll use a placeholder value for this:

UPDATE Customers
SET PhoneNumber = '000-000-0000'

Next, we can alter the phone column to disallow non-null values in SQL Server:


With these steps, we've ensured that every customer in our table has a phone number listed.


The process to alter column null not null in SQL Server involves understanding NULL values, using the correct SQL Server ALTER TABLE syntax, and updating table with NULL values. Moreover, it requires careful planning and execution to avoid SQL Server non-null values errors and ensure data integrity.