Bitkernel·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Did a technical screen for a Software Engineer role at Bitkernel. One question on relational database fundamentals, pretty quick.

Questions Asked (1)

Which of the following statements about primary keys in a relational database is incorrect? (a) Every column in the primary key must be NOT NULL. (b) Every column in the primary key must be unique. (c) A table can have only one primary key. (d) Primary keys can be defined at the column level or table level.

Data ModelingTechnical Trade-offs

Author's notes

The answer is B and I second-guessed myself for a bit.

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AI HintsAI Generated

Suggested Approach

Evaluate each statement against the formal definition of a primary key in the relational model, identifying the one that misstates a requirement. Recall that a primary key is a minimal set of columns that uniquely identifies each row and enforces entity integrity. The incorrect statement is (b) because uniqueness applies to the combination of columns, not to each column individually.

Pro tip: When answering, explicitly distinguish between 'unique' as a property of the entire key versus 'unique' as a property of each column, and mention that this nuance is a common source of confusion in database design discussions.

1. Recall the definition of a primary key

A primary key is a column or set of columns that uniquely identifies each row in a table and cannot contain NULL values.

2. Analyze each statement for correctness

Check each option against the definition: (a) NOT NULL is required for all key columns; (b) uniqueness is required for the key as a whole, not each column; (c) only one primary key per table; (d) primary keys can be defined at column or table level.

3. Identify the incorrect statement

Statement (b) is incorrect because it claims every column in the primary key must be unique, which is false for composite keys where individual columns may have duplicates.

4. Provide a clear explanation and example

Explain why (b) is wrong and give an example of a composite primary key where individual columns are not unique, such as (OrderID, ProductID) in an order details table.

Key Points to Mention

Primary key columns must be NOT NULL (entity integrity).
Uniqueness applies to the combination of columns in a composite primary key, not to each column individually.
A table can have only one primary key, but it can be composite.
Primary keys can be defined at the column level or table level.
Composite primary keys are common in many-to-many relationship tables.
The incorrect statement is (b).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.