The answer is the primary key. The EmployeeID represents this concept because a primary key is the unique identifier for each row in a relational database table, enforcing entity integrity by guaranteeing that every record has a distinct, non-null value. In the context of the Microsoft Azure Data Fundamentals DP-900 exam, this question tests your understanding of fundamental relational database concepts, specifically how primary keys ensure data uniqueness and prevent duplicate records. A common trap on the exam is confusing a primary key with a foreign key or a unique constraint—remember that a primary key uniquely identifies a record within its own table, while a foreign key links to a primary key in another table. For a quick memory tip, think of a primary key as the database’s “social security number” for each row: no two rows can share it, and every row must have one.
DP-900 Describe core data concepts Practice Question
This DP-900 practice question tests your understanding of describe core data concepts. 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 company stores employee records in a database. Each employee record contains an EmployeeID (unique), Name, Department, and HireDate. The EmployeeID is used to uniquely identify each employee. Which data concept does the EmployeeID represent?
Why wrong: An index is a structure that speeds up data retrieval but does not enforce uniqueness or identify rows.
B
Foreign key
Why wrong: A foreign key links a column in one table to the primary key in another table to enforce referential integrity.
C
Primary key
The primary key uniquely identifies each row and is a fundamental concept in relational databases.
D
Unique constraint
Why wrong: A unique constraint ensures that all values in a column are distinct, but unlike a primary key, it allows one NULL value and does not inherently identify a row.
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 EmployeeID is used to uniquely identify each employee record, which is the defining characteristic of a primary key. In relational databases, a primary key enforces entity integrity by ensuring each row has a unique, non-null identifier. This aligns with the core data concept of a primary key as the unique identifier for a table.
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.
✗
Index
Why it's wrong here
An index is a structure that speeds up data retrieval but does not enforce uniqueness or identify rows.
✗
Foreign key
Why it's wrong here
A foreign key links a column in one table to the primary key in another table to enforce referential integrity.
✓
Primary key
Why this is correct
The primary key uniquely identifies each row and is a fundamental concept in relational databases.
Related concept
Read the scenario before looking for a memorised answer.
✗
Unique constraint
Why it's wrong here
A unique constraint ensures that all values in a column are distinct, but unlike a primary key, it allows one NULL value and does not inherently identify a row.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse a unique constraint with a primary key because both enforce uniqueness, but the primary key uniquely identifies the row and cannot contain NULLs, while a unique constraint is a secondary uniqueness enforcement that can allow a single NULL value.
Detailed technical explanation
How to think about this question
Under the hood, a primary key in SQL Server automatically creates a unique clustered index (unless specified as nonclustered), which physically orders the data pages by the key value. This means that queries filtering by EmployeeID can use a seek operation for O(log n) performance. A subtle behavior is that while a unique constraint also creates a unique index, it allows NULL values (though only one NULL in SQL Server), whereas a primary key column is implicitly defined as NOT NULL.
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.
Describe core data concepts — This question tests Describe core data concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Primary key — The EmployeeID is used to uniquely identify each employee record, which is the defining characteristic of a primary key. In relational databases, a primary key enforces entity integrity by ensuring each row has a unique, non-null identifier. This aligns with the core data concept of a primary key as the unique identifier for a table.
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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