In relational databases, what are constraints and which of the following correctly lists common types of integrity constraints?

Introduction / Context:
Constraints are a foundational concept in relational database design. They define rules that the database engine enforces to maintain data integrity, such as ensuring that primary keys are unique or that foreign keys correctly reference parent rows. Interviewers often ask candidates to list the main types of constraints to check basic understanding of how relational databases protect data consistency at the schema level.

Given Data / Assumptions:
• The environment is a relational database such as SQL Server, Oracle, MySQL, or PostgreSQL. • The question asks for both the definition of constraints and examples of common constraint types. • We assume knowledge of standard integrity constraints such as NOT NULL, PRIMARY KEY, and FOREIGN KEY. • No calculations are needed; the problem is purely conceptual.

Concept / Approach:
A constraint is a rule attached to a table or column that restricts the kinds of data that can be stored. NOT NULL ensures that a column cannot store NULL values. UNIQUE enforces that all values in a column or combination of columns are different. PRIMARY KEY combines uniqueness with the requirement that values cannot be NULL and acts as the main row identifier. FOREIGN KEY enforces referential integrity by linking child rows to parent rows. CHECK allows custom logical conditions, and DEFAULT supplies automatic values when none are provided. A correct answer must describe constraints as integrity rules and list these common types accurately.

Step-by-Step Solution:
Step 1: Recall the formal purpose of constraints: enforcing data integrity rules at the database level. Step 2: List standard constraint types: NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT. Step 3: Compare each option to see which one both defines constraints correctly and lists these specific types. Step 4: Observe that option a precisely matches the definition and the known set of common constraints. Step 5: Confirm that the other options actually describe different features, such as backup strategies or indexes, making them incorrect.

Verification / Alternative check:
Think of a typical CREATE TABLE statement that includes constraints. You might define an ID column as PRIMARY KEY, a Name column as NOT NULL, a Code column as UNIQUE, a ParentId column with a FOREIGN KEY reference, and a Status column with a CHECK constraint and a DEFAULT value. Each of these examples corresponds to the standard constraint types listed in option a. The other options mention backup types, index types, logins, or filegroups, none of which are the usual categories of constraints. This confirms that option a is the only accurate description.

Why Other Options Are Wrong:
Option b describes backup types (FULL, DIFFERENTIAL, LOG) which relate to disaster recovery, not constraints. Option c refers to clustered and nonclustered indexes, which are performance structures, not integrity rules. Option d mentions user accounts (Windows login, SQL login), which control authentication and authorization, not data integrity. Option e talks about filegroups (primary and secondary), which define physical storage layout rather than logical integrity rules.

Common Pitfalls:
A common pitfall is underestimating the role of constraints and trying to enforce all rules in application code only. This can lead to inconsistent data if multiple applications write to the database. Another mistake is confusing constraints with indexes; while indexes may be created to support constraints like PRIMARY KEY or UNIQUE, they serve different purposes. It is also important to design constraints thoughtfully because overly strict or poorly chosen rules can make legitimate data impossible to store. However, properly defined constraints are essential for long term data quality and consistency.

Final Answer:
The correct choice is Constraints are rules that enforce data integrity; common types are NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT., because it correctly defines constraints and lists the standard types used to protect relational data integrity.