SQL DISTINCT Clause

The DISTINCT keyword in SQL is used to return unique values from a column or a combination of columns. It helps remove duplicates in query results, making it useful in reporting, analytics, and data cleaning.

📌 Syntax

SELECT DISTINCT column1, column2, ...
FROM table_name
WHERE condition;

📊 Example Table: users

id | name    | age | city
---+---------+-----+---------
1  | Alice   | 22  | New York
2  | Bob     | 30  | London
3  | Charlie | 18  | Sydney
4  | Diana   | 27  | Toronto
5  | Ethan   | 35  | Paris
6  | Frank   | 28  | London
7  | Grace   | 24  | Paris

🔍 Example 1: Distinct Cities

Retrieve all unique cities from the users table:

SELECT DISTINCT city FROM users;

🔍 Example 2: Distinct Names and Cities

Retrieve unique combinations of name and city:

SELECT DISTINCT name, city FROM users;

🔍 Example 3: Using DISTINCT with COUNT

Count the number of unique cities:

SELECT COUNT(DISTINCT city) AS unique_cities FROM users;

🔍 Example 4: DISTINCT with WHERE

Get unique cities where age is greater than 25:

SELECT DISTINCT city 
FROM users 
WHERE age > 25;

⚡ Best Practices

• Use DISTINCT when you need only unique rows to reduce redundancy.
• Combine DISTINCT with COUNT to find the number of unique entries.
• Avoid using DISTINCT unnecessarily on large tables as it may impact performance.
• Use ORDER BY with DISTINCT to sort unique results.

📝 Summary

The DISTINCT keyword is a powerful tool in SQL to eliminate duplicate data and retrieve unique values. It is often combined with aggregation functions, WHERE conditions, and ORDER BY clauses to generate clean and meaningful results from datasets. Understanding how to use DISTINCT efficiently is crucial for reporting, analytics, and data manipulation tasks.

🚀 Next Steps

Next, we will explore the BETWEEN Operator, which allows filtering rows within a specific range of values.