FC0-U71 Data and Database Fundamentals Practice Question
In a relational database, which constraint ensures that a foreign key value matches an existing primary key value in the referenced table?
⚠ Common exam trap
Candidates often confuse the roles of primary key and foreign key constraints: candidates often think the primary key enforces referential integrity, but it only ensures uniqueness within its own table; the foreign key is the one that references the primary key in another table.
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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Foreign key constraint
A foreign key constraint enforces referential integrity by requiring that any value in the foreign key column(s) must either match a primary key or unique key value in the referenced table, or be NULL. This ensures that relationships between tables remain consistent and prevents orphaned records. The constraint is defined on the child table and references the parent table's primary key (or a unique key).
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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Foreign key constraint
Why this is correct
A foreign key constraint enforces referential integrity by rejecting any insert or update whose foreign key value lacks a matching primary key in the referenced table. This directly satisfies the stem's requirement that every foreign key value correspond to an existing primary key, preventing orphaned rows.
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Primary key constraint
Why it's wrong here
A primary key constraint enforces uniqueness and non-null values on the referenced table's own column; it does not validate incoming foreign key values. The foreign key constraint performs that referential check. Primary keys are tempting because they define the referenced target, but they govern the parent table, not the child's reference.
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Check constraint
Why it's wrong here
A check constraint validates values within a single row against a Boolean expression, such as a range or format; it cannot query another table to confirm a matching primary key. Check constraints suit domain validation like age limits. Referential integrity across tables is enforced by the foreign key constraint instead.
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Unique constraint
Why it's wrong here
A unique constraint enforces distinctness within a single column or set of columns; it does not reference another table, so it cannot validate that a foreign key value exists in the parent table. It is tempting because both constraints police column values, but uniqueness applies within one table, whereas referential integrity requires the foreign key constraint.
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Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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