SQL Unique

What does the UNIQUE constraint do in SQL?

The UNIQUE constraint in SQL ensures that all values in a column are distinct. It prevents duplicate entries, maintaining data integrity and consistency. This is crucial for columns that represent unique identifiers or characteristics.

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Description

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Example H3

The UNIQUE constraint in SQL is a powerful tool for maintaining data integrity within a database. It ensures that no two rows in a table have identical values in a specified column. This is particularly important when dealing with columns that represent unique identifiers, like usernames, product codes, or customer IDs. By enforcing uniqueness, the UNIQUE constraint prevents data inconsistencies and errors that could arise from duplicate entries. This constraint is a fundamental aspect of relational database design, as it helps maintain the accuracy and reliability of the data stored within the database. For example, if you have a table of employees, a UNIQUE constraint on the employee ID column ensures that no two employees have the same ID. This constraint is enforced by the database management system (DBMS) and automatically rejects any attempt to insert a duplicate value into the column. It's important to note that the UNIQUE constraint differs from a PRIMARY KEY constraint, which also enforces uniqueness but also acts as the primary identifier for the table.

Why SQL Unique is important

UNIQUE constraints are vital for maintaining data accuracy and consistency in databases. They prevent data redundancy and ensure that each row in a table is uniquely identifiable. This is crucial for applications that rely on accurate data, such as e-commerce platforms, inventory management systems, and customer relationship management (CRM) systems.

SQL Unique Example Usage


-- Creating a temporary table
CREATE TABLE #temp_products (
    ProductID INT,
    ProductName VARCHAR(50),
    Price DECIMAL(10, 2)
);

-- Inserting data into the temporary table
INSERT INTO #temp_products (ProductID, ProductName, Price)
VALUES
(1, 'Laptop', 1200.00),
(2, 'Mouse', 25.00),
(3, 'Keyboard', 75.00);

-- Selecting data from the temporary table
SELECT * FROM #temp_products;

-- Calculating the total price of all products
SELECT SUM(Price) AS TotalPrice FROM #temp_products;

-- Dropping the temporary table (automatic when session ends)
DROP TABLE #temp_products;

SQL Unique Syntax

Common Mistakes

Forgetting to define the UNIQUE constraint when it's necessary.
Using UNIQUE constraints on columns that are not intended to be unique.
Confusing UNIQUE constraints with PRIMARY KEY constraints.
Not understanding the implications of violating a UNIQUE constraint.

Frequently Asked Questions (FAQs)

How does a UNIQUE constraint differ from a PRIMARY KEY constraint?

Both constraints enforce that every value in the column (or column set) is distinct, but a PRIMARY KEY goes a step further by serving as the table’s main identifier and automatically creating a clustered or non-clustered index (depending on the RDBMS). A UNIQUE constraint guarantees no duplicates yet allows multiple NULLs (in most databases) and does not imply that the column is the primary identifier. In short, every PRIMARY KEY is unique, but not every UNIQUE constraint is a primary key.

Which columns benefit most from a UNIQUE constraint?

Columns that represent real-world identifiers—usernames, email addresses, employee IDs, product SKUs, or customer numbers—should be protected by a UNIQUE constraint. Doing so prevents data anomalies, stops accidental duplicate inserts, and ensures downstream applications can safely rely on those columns as stable look-ups or joins.

How can Galaxy’s AI SQL editor help when working with UNIQUE constraints?

Galaxy’s context-aware AI copilot can automatically detect existing UNIQUE constraints from your table metadata, warn you when a draft INSERT might violate uniqueness, and even suggest adding a constraint when it spots duplicate-prone columns. This proactive guidance speeds up development, avoids runtime errors, and keeps your schema in sync across teammates who share queries through Galaxy’s Collections.