easyMultiple Choice
FC0-U71 Practice Question: A database designer wants to ensure that no two…
A database designer wants to ensure that no two employees have the same email address. Which constraint should be applied to the Email column?
⚠ Common exam trap
A common mix-up: candidates confuse UNIQUE with PRIMARY KEY, assuming uniqueness can only be enforced by a primary key, but PRIMARY KEY imposes additional non-null and single-per-table restrictions that are not required for simply ensuring unique email addresses.
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
✓
UNIQUE
The UNIQUE constraint ensures that all values in the Email column are distinct, preventing any two employees from having the same email address. Unlike PRIMARY KEY, UNIQUE allows NULL values (though typically email columns are set to NOT NULL), and it can be applied to non-key columns. This directly enforces the business rule of unique email addresses without requiring the column to be the table's primary identifier.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
UNIQUE
Why this is correct
A UNIQUE constraint enforces distinct values in the Email column, rejecting any duplicate insert or update at the database engine level. This directly prevents two employee rows sharing an email address, satisfying the designer's requirement without relying on application-side validation.
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PRIMARY KEY
Why it's wrong here
A primary key enforces uniqueness and non-null values, but a table permits only one, and it is normally the employee identifier rather than the email column. It tempts because it does guarantee uniqueness, yet the requirement targets a single non-key column.
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CHECK
Why it's wrong here
A CHECK constraint validates each row against a condition such as a format pattern; it cannot enforce uniqueness across rows. It tempts because it constrains column values, but preventing duplicate emails requires a UNIQUE constraint, which the CHECK predicate cannot provide.
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FOREIGN KEY
Why it's wrong here
A foreign key enforces referential integrity by requiring values to exist in a referenced table's primary key, so it cannot prevent duplicate emails within the Email column itself. It is tempting because foreign keys do link tables, but that is for relational consistency, not uniqueness.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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