In SQL Server or similar systems, what is a DEFAULT value and is there any type of column to which a default cannot normally be bound?

Introduction / Context:
Defaults are part of the data integrity and convenience features of relational databases such as SQL Server. They allow the database to automatically provide a value for a column when an INSERT statement does not specify one. This question also touches on an important limitation: some special columns, such as identity or timestamp columns, are automatically generated by the system and therefore do not accept user supplied defaults. Understanding defaults is important for designing tables and controlling how missing data is handled.

Given Data / Assumptions:
• The environment is SQL Server or a similar relational database with DEFAULT constraints. • The question asks both what a default is and whether any column type cannot normally have a default bound to it. • We assume familiarity with system generated columns like IDENTITY and TIMESTAMP (or ROWVERSION). • No calculations are needed; the focus is on conceptual understanding of schema design.

Concept / Approach:
A DEFAULT in SQL is a value that the database engine inserts into a column when a row is created and the INSERT statement omits that column. It can be defined using a literal or an expression. Defaults are implemented as constraints in many systems. However, some columns are controlled entirely by the database engine, for example IDENTITY columns that automatically increment or TIMESTAMP columns that store versioning information. For these system managed columns, the database ignores or disallows user supplied defaults, because their values are not intended to be set manually. A correct answer must capture both the automatic value concept and the limitation on system generated columns.

Step-by-Step Solution:
Step 1: Recall that when an INSERT does not mention a column, the database will either use a default, allow NULL, or raise an error depending on the schema. Step 2: Recognize that defining a DEFAULT constraint ensures a sensible automatic value, such as 0, GETDATE(), or a status code. Step 3: Identify columns like IDENTITY or TIMESTAMP whose values are generated by the system engine rather than by user input. Step 4: Understand that binding a default to these system generated columns does not make sense, because the database controls their values. Step 5: Choose the option that correctly describes a default as an automatic value and states that system generated columns typically cannot have user defined defaults.

Verification / Alternative check:
Consider a table with an IDENTITY primary key column and a CreatedDate column with a default of GETDATE(). When inserting a new row and omitting both columns, SQL Server automatically generates the next identity value and uses the current date and time as the default CreatedDate. Defining a default on the IDENTITY column would be redundant or invalid, because SQL Server already generates that value. Similarly, TIMESTAMP or ROWVERSION columns are automatically updated whenever the row changes and do not accept explicit defaults. This confirms that defaults apply to regular columns but not to those fully controlled by the database engine.

Why Other Options Are Wrong:
Option b misidentifies a default as an index, which is a separate structure for speeding up queries, not a value. Option c states that a default is a backup copy of each row, which is completely unrelated to the DEFAULT feature. Option d confuses defaults with foreign keys and adds an incorrect restriction about character columns. Option e incorrectly defines a default as a NULL marker that replaces every value, which misunderstands both NULL handling and default behavior.

Common Pitfalls:
A common pitfall is assuming that every column should have a default, which may hide data quality issues when columns should instead require explicit values. Another mistake is forgetting that defaults only apply when the column is omitted, not when NULL is explicitly inserted. Developers also sometimes assume they can override system generated columns with defaults or manual values; in practice, identity and timestamp columns are tightly controlled by the engine. Properly distinguishing between user defined defaults and system managed values is essential for predictable behavior.

Final Answer:
The correct choice is A default is a value automatically supplied by the database when no explicit value is provided for a column; defaults cannot be bound to columns whose values are automatically generated by the system such as IDENTITY or TIMESTAMP., because it accurately describes the purpose of defaults and the limitation on system controlled columns.