The answer is the PRIMARY KEY constraint. This is the correct choice because a primary key is the only database constraint that inherently enforces both uniqueness and non-nullability on a column or set of columns, ensuring that every value is distinct and cannot be left empty. In the context of the Microsoft Azure Data Fundamentals DP-900 exam, this question tests your understanding of fundamental relational database concepts, specifically how constraints like PRIMARY KEY, UNIQUE, and NOT NULL differ. A common trap is confusing the UNIQUE constraint, which allows one NULL value, with the PRIMARY KEY, which rejects all NULLs. For the exam, remember that any column that must be both unique and non-null—such as a patient's medical record number—is a natural candidate for a primary key. A simple memory tip: think of a primary key as the "golden ID"—it must be there and it must be one of a kind.
DP-900 Practice Question: Identify considerations for relational data on Azure
This DP-900 practice question tests your understanding of identify considerations for relational data on azure. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A development team is designing a relational database for a hospital patient management system. They need to ensure that each patient's medical record number is unique and not null. Which database constraint should they use?
Why wrong: Incorrect. FOREIGN KEY constraints are used to enforce relationships between tables, not to ensure uniqueness or non-null on a single column.
B
CHECK
Why wrong: Incorrect. CHECK constraints validate that data meets a Boolean expression, but they do not inherently enforce uniqueness.
C
UNIQUE
Why wrong: Incorrect. A UNIQUE constraint guarantees uniqueness but allows one null value in most database systems. The requirement specifies non-null as well.
D
PRIMARY KEY
Correct. A PRIMARY KEY constraint enforces both uniqueness and non-null, making it ideal for the medical record number.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PRIMARY KEY
The PRIMARY KEY constraint enforces both uniqueness and non-nullability on the column(s) it is applied to. In a relational database, the medical record number is the natural candidate for the primary key of the Patient table because it uniquely identifies each patient and must always have a value. This directly meets the requirement that each patient's medical record number is unique and not null.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
FOREIGN KEY
Why it's wrong here
Incorrect. FOREIGN KEY constraints are used to enforce relationships between tables, not to ensure uniqueness or non-null on a single column.
✗
CHECK
Why it's wrong here
Incorrect. CHECK constraints validate that data meets a Boolean expression, but they do not inherently enforce uniqueness.
✗
UNIQUE
Why it's wrong here
Incorrect. A UNIQUE constraint guarantees uniqueness but allows one null value in most database systems. The requirement specifies non-null as well.
✓
PRIMARY KEY
Why this is correct
Correct. A PRIMARY KEY constraint enforces both uniqueness and non-null, making it ideal for the medical record number.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse UNIQUE with PRIMARY KEY, forgetting that UNIQUE allows NULL values (in most RDBMS implementations) and therefore does not satisfy the 'not null' requirement without an additional NOT NULL constraint.
Detailed technical explanation
How to think about this question
Under the hood, a PRIMARY KEY constraint automatically creates a unique clustered index (by default in SQL Server) on the column(s), which physically organizes the data rows in that order and enforces both uniqueness and non-nullability at the storage engine level. A subtle behavior is that while a UNIQUE constraint also creates an index, it permits one NULL value per column (in SQL Server), which would violate the 'not null' requirement. In a real-world hospital system, the medical record number is often used as the primary key to ensure fast lookups and to serve as the target for foreign key references from related tables like Visits or Prescriptions.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this DP-900 question in full detail.
Identify considerations for relational data on Azure — This question tests Identify considerations for relational data on Azure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: PRIMARY KEY — The PRIMARY KEY constraint enforces both uniqueness and non-nullability on the column(s) it is applied to. In a relational database, the medical record number is the natural candidate for the primary key of the Patient table because it uniquely identifies each patient and must always have a value. This directly meets the requirement that each patient's medical record number is unique and not null.
What should I do if I get this DP-900 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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