SQL Count Unique

How do you count unique values in a SQL table?

The `COUNT(DISTINCT column)` function in SQL is used to count the unique values within a specified column. It's crucial for getting a precise count of distinct items, avoiding duplicates. This is essential for tasks like finding the number of unique customers or products.

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Description

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The `COUNT(DISTINCT column)` function in SQL is a powerful tool for counting unique values in a specific column of a table. Unlike the standard `COUNT()` function, which counts all rows, `COUNT(DISTINCT column)` only counts each unique value once. This is particularly useful when you need to know the number of distinct items, such as the number of unique product names, customer IDs, or order numbers. It's a fundamental aggregate function used in various database queries. For example, in an e-commerce database, you might want to know the total number of unique products sold. Using `COUNT(DISTINCT product_name)` would give you that precise count. This function is also crucial for data analysis, reporting, and business intelligence tasks. It helps in understanding the variety and breadth of data within a dataset.

Why SQL Count Unique is important

The `COUNT(DISTINCT)` function is vital for accurate data analysis. It allows for precise counts of unique items, which is essential for understanding the variety and breadth of data. This is crucial for reporting, business intelligence, and decision-making.

SQL Count Unique Example Usage


-- Creating a table
CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    City VARCHAR(50)
);

-- Inserting data into the table
INSERT INTO Customers (CustomerID, FirstName, LastName, City)
VALUES
(1, 'John', 'Doe', 'New York'),
(2, 'Jane', 'Smith', 'Los Angeles');

-- Retrieving data from the table
SELECT * FROM Customers;

-- Updating data in the table
UPDATE Customers
SET City = 'Chicago'
WHERE CustomerID = 2;

-- Deleting data from the table
DELETE FROM Customers
WHERE CustomerID = 1;

SQL Count Unique Syntax

Common Mistakes

Forgetting to use `DISTINCT` when you need a unique count, leading to incorrect results.
Using `COUNT(*)` when you need a unique count, which will count all rows, including duplicates.
Incorrectly specifying the column for `COUNT(DISTINCT)` which will result in an error or an incorrect count.

Frequently Asked Questions (FAQs)

When should I use `COUNT(DISTINCT)` instead of `COUNT(*)` in SQL?

Use COUNT(DISTINCT column) when you need the number of unique values in a column—such as distinct product names, customer IDs, or order numbers—instead of the total row count. Unlike COUNT(*), which tallies every row (including duplicates), the DISTINCT modifier ensures each value is counted only once, giving you an accurate picture of variety within your data set.

Can I count unique combinations across multiple columns?

Yes, most modern SQL engines allow COUNT(DISTINCT (col1, col2)) or list syntax like COUNT(DISTINCT col1 || '-' || col2) to measure unique pairs or tuples—e.g., unique customer product combinations. Syntax support differs by database (PostgreSQL and MySQL support tuple-style COUNT(DISTINCT col1, col2)), so check your engine’s documentation or use a concatenation workaround where needed.

How does Galaxy help me write and optimize `COUNT(DISTINCT)` queries faster?

Galaxy’s lightning-fast SQL editor and context-aware AI copilot auto-complete table names, suggest the correct COUNT(DISTINCT ...) syntax for your dialect, and even warn you if a query could be optimized with indexes or a subquery. You can iterate quickly without ballooning memory, share endorsed queries with teammates, and keep everyone aligned on distinct-count metrics—all directly inside Galaxy’s desktop or cloud workspace.