DP-900 Describe core data concepts Practice Question
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?
⚠ Common exam trap
Many exam-takers 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.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Index
Why it's wrong here
An index is a separate on-disk structure that accelerates data access by enabling the database engine to locate rows quickly through ordered key values. Although a primary key is often automatically backed by a unique clustered or nonclustered index, an index itself is only a performance optimization and does not define row identity, enforce a uniqueness business rule, or guarantee that every employee appears only once. Therefore, creating an index on employee data would not ensure each employee record can be uniquely identified.
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Foreign key
Why it's wrong here
A foreign key is a referential integrity constraint that ties a column in one table to the primary key (or a unique key) of another table, ensuring that a value such as DepartmentID corresponds to an existing department. It is designed to preserve relationships between related records, not to make rows within the employee table unique. An employee table could have several foreign keys—for manager, department, or job code—and still contain duplicate employee rows, so a foreign key cannot serve as the column that uniquely identifies each employee.
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Primary key
Why this is correct
A primary key is the column or set of columns declared to hold a unique, non-null value for every row, and it serves as the table's authoritative identifier for each entity record. For an employee table, an EmployeeID column or a composite of first name, last name, and birth date would be a primary key because it guarantees that each employee row can be individually referenced, updated, or joined to related tables. This is the fundamental relational mechanism for row identity and is what other tables use when they need to reference a specific employee.
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Unique constraint
Why it's wrong here
A unique constraint prevents duplicate values in a column or combination of columns, but it is not the table's designated row identifier. In SQL Server, a unique constraint permits at most one NULL value, so it cannot guarantee that every employee row has a usable identifier. It is typically used for alternate keys such as email addresses or badge numbers, which may be distinct yet still not represent the logical identity that a primary key represents.
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Related to this question
Learn chapter
Data Roles and Core Concepts
Key term
Primary key
A primary key is a unique identifier for each record in a database table, ensuring that no two rows have the same value in that column.
Key term
Data
Data is raw, unprocessed information, like numbers, words, or measurements, that can be stored, processed, and analyzed by computers.
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