Change a Nullable column to NOT NULL with Default Value

Data Consistency and Backward Compatibility

Changing a column from Nullable to NOT NULL can bring along troublesome side effects. An application's logic that previously tolerated Nullable values may hiccup on encountering the fresh NOT NULL stipulation. So before going ahead, critically examine the behavior of your applications interfacing with the database.

Likewise, remember to backup the data prior to any changes to the schema to effectively mitigate any risks of data loss. The easiest method in SQL Server to backup a table is:

SELECT *
INTO backup_your_table // Just in case!
FROM your_table;
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Default Value Insertion: The Automated Edition

A popular implementation example is when a datetime default value, such as the current date-time, is required every time a row gets added:

ALTER TABLE your_table
ADD CONSTRAINT DF_your_column_default
DEFAULT (getdate()) FOR your_column; // Every day is special!
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Verify if the datatype_alignment complements the default value. Particularly with dates and times, things can get messy due to diverse ways of formatting.

Diverse SQL Environments: Different Strokes

The syntax to alter tables sprawls across SQL flavors.

In MySQL, use:

ALTER TABLE your_table
MODIFY your_column VARCHAR(255) NOT NULL DEFAULT 'default_value'; // MySQL style!
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Alternatively, in PostgreSQL, the syntax differs:

ALTER TABLE your_table
ALTER COLUMN your_column SET NOT NULL,
ALTER COLUMN your_column SET DEFAULT 'default_value'; // PostgreSQL flair!
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When juggling multiple SQL dialects, consultation with the relevant documentation becomes even more critical.

Default Constraints: The Unique Case

Sometimes, a unique identifier like a GUID is needed as the default value. You would utilize functions like NEWID() or UUID() in such scenarios:

ALTER TABLE your_table
ADD CONSTRAINT DF_your_column_default
DEFAULT (NEWID()) FOR your_column; // Each column has its own identity!
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