An unique (non-key) field?

Peer-reviewed by HireXTech Technical Panel • Updated for 2025/2026 hiring • Editorial standards

Interviewer Expectations for this Question

Core Competency

Assesses fundamental understanding of SQL & Databases conventions, runtime behavior, and memory/performance considerations.

Evaluation Criteria

Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.

Comprehensive Model Answer Verified Solution

To enforce that every value in a non-primary key column is unique, apply the UNIQUE constraint:

CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(255) UNIQUE NOT NULL,
    phone_number VARCHAR(20) UNIQUE
);

### PRIMARY KEY vs UNIQUE:

A table can have only one PRIMARY KEY.
A table can have multiple UNIQUE constraints.
UNIQUE columns typically allow NULL values (in SQL standards, multiple NULL values are permitted because NULL != NULL).

Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.

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