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

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

Efficiently managing SQL Server databases often requires adding columns to existing tables, especially when a predefined default value is needed for data integrity and streamlined data handling. This guide explains how to accomplish this in SQL Server 2000 and 2005.

Adding a Column with a Default Value: Syntax and Example

The following SQL syntax adds a column with a default value:

<code class="language-sql">ALTER TABLE {TABLENAME}
ADD {COLUMNNAME} {TYPE} {NULL | NOT NULL}
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES;</code>

Here's a practical example:

<code class="language-sql">ALTER TABLE SomeTable
ADD SomeCol BIT NULL --  Specify NULL or NOT NULL as needed.
CONSTRAINT D_SomeTable_SomeCol -- Optional constraint name.
DEFAULT (0) -- Optional default value.
WITH VALUES; -- Essential if the column allows NULLs and you want to apply the default to existing rows.</code>

Important Considerations

• Constraint Naming: Leaving out CONSTRAINT D_SomeTable_SomeCol results in SQL Server automatically generating a more complex default constraint name.
• WITH VALUES Clause: This clause is crucial only when the column permits NULL values and you intend to apply the default value to existing rows. For NOT NULL columns, the default value is automatically applied to existing rows regardless of this clause.
• Data Insertion Behavior: If SomeCol is omitted during data insertion, it defaults to 0. However, if SomeCol is defined as NULL and allows NULLs, the default constraint is bypassed, and NULL is inserted.

Summary

Adding a column with a default value to an existing SQL Server table is a simple yet powerful technique. Following the provided syntax and guidelines ensures data integrity and simplifies database management.

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