SQL INTERVIEW PREPARATION PART-16

Question 16: What is the difference between a primary key and a unique key in SQL?

Answer:
- Primary Key:
- Uniquely identifies each record in a table.
- Cannot contain NULL values.
- A table can have only one primary key.
- Often automatically creates a clustered index on the column(s).

- Unique Key:
- Ensures that all values in a column or a set of columns are unique across the table.
- Can contain NULL values, but each NULL is considered unique.
- A table can have multiple unique keys.
- Often creates a non-clustered index on the column(s).

Example:
CREATE TABLE employees (
id INT PRIMARY KEY,
email VARCHAR(255) UNIQUE
);
In this example, id is the primary key, ensuring no duplicate values and no NULLs, while email is a unique key, ensuring unique values but allowing NULLs.

Tip: Clarify that the primary key is a more stringent constraint than a unique key because it does not allow NULL values. Emphasize the importance of both constraints in maintaining data integrity and ensuring that records can be uniquely identified.

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