Select Distinct SQL

How do you retrieve unique rows from a table in SQL?

The `SELECT DISTINCT` clause in SQL is used to retrieve only unique rows from a table. It eliminates duplicate rows, returning each distinct combination of values in the specified columns. This is crucial for data analysis and reporting.

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Description

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The `SELECT DISTINCT` clause is a powerful tool in SQL for filtering out duplicate rows from a result set. When you query a table, you might get multiple rows with identical values in certain columns. `SELECT DISTINCT` ensures that you only see one copy of each unique combination of values. This is particularly useful when you need to identify unique customers, products, or any other entity represented in your database. It's a fundamental part of data manipulation, enabling you to work with clean, non-redundant data. Imagine a table listing orders. Without `SELECT DISTINCT`, you might see multiple rows for the same customer's order. Using `SELECT DISTINCT` on the customer ID column would show you only one row per customer, simplifying analysis and reporting. It's important to note that `SELECT DISTINCT` considers all selected columns. If any column has a different value, it's considered a distinct row. For example, if you have two orders with the same customer ID but different order dates, `SELECT DISTINCT` will treat them as distinct rows.

Why Select Distinct SQL is important

The `SELECT DISTINCT` clause is essential for data integrity and analysis. It helps to avoid redundant data, making queries more efficient and results easier to interpret. It's a fundamental tool for data cleaning and preparation, crucial for accurate reporting and decision-making.

Select Distinct SQL Example Usage


-- Sample table (Customers)
CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Email VARCHAR(100)
);

-- Insert some data, including a NULL value
INSERT INTO Customers (CustomerID, FirstName, LastName, Email)
VALUES
(1, 'John', 'Doe', 'john.doe@example.com'),
(2, 'Jane', 'Smith', NULL),
(3, 'Peter', 'Jones', 'peter.jones@example.com');

-- Query to retrieve customers with a valid email address
SELECT CustomerID, FirstName, LastName, Email
FROM Customers
WHERE Email IS NOT NULL;

Select Distinct SQL Syntax

Common Mistakes

Forgetting to use `DISTINCT` when you need unique values, leading to redundant data.
Using `DISTINCT` on columns that aren't relevant to identifying unique rows, resulting in unexpected or incorrect results.
Not understanding that `DISTINCT` considers all selected columns, potentially missing unique combinations if not all relevant columns are included.

Frequently Asked Questions (FAQs)

When is `SELECT DISTINCT` the right choice for removing duplicates?

Use SELECT DISTINCT whenever you need a quick way to eliminate duplicate rows and view only one record for each unique combination of the selected columns. For example, if your orders table has multiple rows for the same customer, running SELECT DISTINCT customer_id FROM orders instantly shows a deduplicated customer list—perfect for reporting, segmentation, or building drop-downs.

What happens if I add more columns to a `SELECT DISTINCT` statement?

The DISTINCT check applies to all columns in the SELECT list. If any additional column has a different value, the entire row is considered unique and will appear in the results. In practice, two orders with the same customer_id but different order_date values will be returned as separate rows because the full combination of selected columns is no longer identical.

How does Galaxy speed up writing and sharing `SELECT DISTINCT` queries?

Galaxy’s AI-powered SQL editor auto-completes keywords like SELECT DISTINCT, suggests column names as you type, and can even generate the full deduplication query for you. Once written, you can add the query to a Collection and endorse it so teammates reuse the exact same logic—no more pasting SQL into Slack or Notion. The context-aware copilot also explains why certain columns make the result distinct, helping you avoid accidental duplicates.