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A Unique Key, in SQL is a restriction that guarantees every value in a column (or combination of columns) is distinct—so no two rows share the value in that attribute.

✅ Definition of Unique Key

A Unique Key guarantees that duplicate entries are prohibited yet NULL values are permitted (although some database systems permit one NULL).

It is utilized to uphold data integrity by avoiding repeated entries.

⭐ Key Characteristics of a Unique Key

✔ 1. Guarantees Distinctiveness

It is not permitted for any two rows to share values, in the unique key column.

✔ 2. Permits NULL Values

The majority of SQL databases (MySQL, PostgreSQL SQL Server) permit one or NULL values within a unique key.

Oracle permits NULL values.

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✔ 3. A Table May Contain Unique Keys

In contrast to a key (limited to one, per table) multiple unique keys can exist within a table.

✔ 4. Not the Primary Identifier (usually)

A primary key distinctively identifies the record; unique keys assist in preserving uniqueness in crucial fields (such, as email, phone number).

📌 Example of Unique Key

Creating a table with a unique key

CREATE TABLE users (

user_id INT PRIMARY KEY,

email VARCHAR(100) UNIQUE,

phone VARCHAR(15) UNIQUE

);

Here:

email must be unique.

phone must be unique.

Each can be NULL (depending on the database engine).

📌 Adding a Unique Constraint Later

ALTER TABLE users

ADD CONSTRAINT unique_email UNIQUE (email);

📌 Unique Key on Multiple Columns (Composite Unique)

ALTER TABLE orders

ADD CONSTRAINT unique_order UNIQUE (customer_id, product_id);

This indicates that a single customer is not allowed to purchase the product more, than once although they are permitted to buy various products.

🔍 Unique Key vs Primary Key

Feature Primary Key Unique Key

Uniqueness ✔ Yes ✔ Yes

NULL Permitted ❌ No ✔ Yes

Quantity Permitted 1, per table Multiple

Purpose Identifies row Enforces uniqueness

If you'd like, I can also explain Foreign Key, Primary Key, or Composite Key with real-life examples!

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