SQL Unique Values

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How do you find and ensure unique values in a SQL table?

Unique values in SQL are values that are distinct within a column. Ensuring uniqueness is crucial for data integrity. SQL provides various ways to identify and enforce this.

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Description

Unique values are essential in databases to avoid redundancy and maintain data accuracy. In a table, a column containing unique values ensures that no two rows have the same value in that specific column. This is vital for representing distinct entities or attributes. For example, in a customer table, the customer ID column should ideally contain unique values to identify each customer uniquely. SQL offers several methods to identify and enforce uniqueness. One common way is using the `DISTINCT` keyword in `SELECT` statements to retrieve only unique values. Another approach involves creating a unique constraint on a column to prevent the insertion of duplicate values during data entry. This constraint is enforced by the database system, ensuring data integrity. Understanding unique values is fundamental for building robust and reliable database applications.

Why SQL Unique Values is important

Unique values are critical for data integrity and consistency. They prevent data duplication, enabling accurate analysis and reporting. They also simplify data management and reduce the risk of errors.

Example Usage


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

-- Insert data into the temporary table
INSERT INTO #tempProducts (ProductID, ProductName, Price)
VALUES
(1, 'Laptop', 1200.50),
(2, 'Mouse', 25.00),
(3, 'Keyboard', 75.00);

-- Select data from the temporary table
SELECT * FROM #tempProducts;

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

-- Drop the temporary table
DROP TABLE #tempProducts;

Common Mistakes

Forgetting to use `DISTINCT` when needing unique values.
Not creating unique constraints to enforce uniqueness.
Using a column that isn't truly unique as a primary key.
Trying to insert duplicate values into a column with a unique constraint.

SQL Unique Values

How do you find and ensure unique values in a SQL table?

Description

Why SQL Unique Values is important

Example Usage

Common Mistakes

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