How to Add a New Column with a Default Value to an Existing SQL Server Table?

Adding a New Column with a Default Value to an Existing SQL Server Table

Modifying existing SQL Server tables frequently involves adding new columns. This process can be easily enhanced by assigning a default value to the new column. This ensures data consistency and simplifies the insertion of new rows.

SQL Syntax:

ALTER TABLE {TABLENAME}
ADD {COLUMNNAME} {TYPE} {NULL | NOT NULL}
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES;

Explanation of Parameters:

• {TABLENAME}: The name of the table you're modifying.
• {COLUMNNAME}: The name you're giving to the new column.
• {TYPE}: The data type of the new column (e.g., INT, VARCHAR(255), BIT).
• {NULL | NOT NULL}: Specifies whether the column can accept NULL values.
• {CONSTRAINT_NAME}: (Optional) A user-defined name for the default constraint. If omitted, SQL Server will generate a name automatically.
• {DEFAULT_VALUE}: The value that will be automatically assigned to the new column if no value is provided during insertion.
• WITH VALUES: (Optional) This clause applies the default value to existing rows in the table only if the column allows NULL values.

Practical Example:

Let's add a "IsActive" column (BIT data type) to a table named "Products," allowing NULLs and setting the default to 1 (true):

ALTER TABLE Products
ADD IsActive BIT NULL
CONSTRAINT DF_Products_IsActive
DEFAULT (1)
WITH VALUES;

Important Considerations:

• Constraint Name: While optional, providing a descriptive constraint name improves readability and maintainability of your database schema.
• WITH VALUES Clause: Use WITH VALUES judiciously. It's only needed when you want pre-existing rows to inherit the default value. Otherwise, existing rows will have NULL in the new column.
• Default Constraint Behavior: The default constraint dictates the value used during insertion only if no value is explicitly provided. If you explicitly insert NULL, the default value is ignored.

This detailed explanation and example should make adding columns with default values in SQL Server a straightforward process.

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