Select Unique SQL

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

The `DISTINCT` keyword in SQL is used to eliminate duplicate rows from a result set, returning only unique values for a specified column or set of columns. This is crucial for data analysis and reporting, ensuring accurate counts and summaries.

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Description

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In SQL, retrieving unique data is a common task. Imagine you have a table of customer orders, and you want to see a list of all the unique products ordered. Using the `DISTINCT` keyword, you can easily achieve this. The `DISTINCT` keyword filters out duplicate rows, ensuring that each row in the result set is unique. This is particularly useful when you need to identify distinct categories, values, or combinations of values within your data. For example, you might want to find the unique cities where your customers reside or the unique product types sold. The `DISTINCT` keyword is a fundamental tool for data analysis and reporting, ensuring that your results are accurate and reliable. It's important to note that `DISTINCT` operates on the entire row, not just a single column. If you want to select unique values from a specific column, you specify that column in the `SELECT` statement. This ensures that only the unique values from that column are returned.

Why Select Unique SQL is important

The `DISTINCT` keyword is essential for accurate data analysis and reporting. It ensures that results are not skewed by duplicate entries, providing a clear picture of the unique values present in your data. This is crucial for tasks like calculating unique customer counts, identifying distinct product types, or generating accurate summaries.

Select Unique SQL Example Usage


-- Find all customers whose names start with 'A'.
SELECT customer_name
FROM Customers
WHERE customer_name LIKE 'A%';

-- Find all products containing the word 'Laptop'.
SELECT product_name
FROM Products
WHERE product_name LIKE '%Laptop%';

-- Find all customers whose names have exactly 5 characters.
SELECT customer_name
FROM Customers
WHERE customer_name LIKE '_____';

-- Find all products whose names contain a space.
SELECT product_name
FROM Products
WHERE product_name LIKE '% %';

-- Find all products whose names start with 'T' and have exactly 7 characters.
SELECT product_name
FROM Products
WHERE product_name LIKE 'T_____';

Select Unique SQL Syntax

Common Mistakes

Forgetting to use `DISTINCT` when unique values are needed, leading to duplicate rows in the result set.
Using `DISTINCT` without specifying the column(s) to filter, potentially returning unexpected results.
Confusing `DISTINCT` with `GROUP BY`, which is used for aggregating data based on groups of rows.

Frequently Asked Questions (FAQs)

When should I use `SELECT DISTINCT` instead of `GROUP BY`?

Use SELECT DISTINCT when your only goal is to eliminate duplicate rows and return a list of unique values (or value combinations) with no aggregations such as SUM() or COUNT(). Because DISTINCT skips the overhead of computing aggregates, it is usually simpler and can be faster for de-duplicating data like a list of unique product IDs, cities, or user emails.

Does `DISTINCT` work on the entire row or only on the specified column?

DISTINCT evaluates uniqueness across all columns returned by the SELECT clause. If you include multiple columns, only rows where the full combination is unique will appear in the result set. To get unique values from just one column, select only that column: SELECT DISTINCT city FROM customers;.

How can Galaxy help me write `DISTINCT` queries faster?

Galaxy’s context-aware AI copilot autocompletes table names, columns, and even suggests adding DISTINCT when it detects you are de-duplicating data. The lightning-fast editor surfaces table metadata inline, so you can confirm which columns form a unique combination without leaving your keyboard—speeding up the creation and validation of SELECT DISTINCT statements.