Sql Query Distinct with Row_Number

Breaking down the basics

Using DENSE_RANK() over ROW_NUMBER()

When the target is to retrieve unique values with their respective row numbers, using DENSE_RANK() can be more effective than ROW_NUMBER(). Why? Let's find out:

SELECT id, DENSE_RANK() OVER (ORDER BY id) AS RowNum
FROM YourTable
-- Team building but GROUP_BY style: id, you're it! 👯‍♀️
GROUP BY id; 


This structure handles cases where the same rank is required for duplicate entries. The ORDER BY clause inside the DENSE_RANK() ensures a suitable ordering of results.

Tackling multiple distinct fields

When dealing with multiple unique fields, you can use ROW_NUMBER() combined with PARTITION BY for an efficient query:

-- 'id', 'description' forming a great band, keeping 'creation_date' up the tempo! 🎻🥁
SELECT id, description, ROW_NUMBER() OVER (PARTITION BY id, description ORDER BY creation_date) AS rn
FROM YourTable;


Diving deeper

Making most of window functions

For better ROW_NUMBER(), RANK(), and DENSE_RANK() usage, look no further than window functions:

-- Sales on steroids! Adding RANK() fuels a friendly competition! 💪
SELECT id, sales, RANK() OVER (ORDER BY sales DESC) AS sales_rank
FROM SalesTable;


The RANK() function gives a unique ranking based on sales, enabling you to track each record's performance.

Simplifying with subqueries and CTEs

Employing subqueries to combine DISTINCT and ROW_NUMBER() turns out to be highly efficient:

SELECT DISTINCT id, sub.rn
FROM (
    -- Prepare, SET, sub-select! 🚀
    SELECT id, ROW_NUMBER() OVER (ORDER BY id) AS rn
    FROM YourTable
) AS sub;


Also, using Common Table Expressions (CTEs), like you saw in the "Fast Answer" section, improves readability, especially for complex queries.

Extracting insights using GROUP BY

For deriving metrics such as max() or COUNT() alongside unique row numbers, GROUP BY with ROW_NUMBER() can yield insightful group-wise results:

-- Courtesy COUNT(*), we now have a fan club for every 'id'! 🎊
SELECT id, COUNT(*) as frequency, ROW_NUMBER() OVER (ORDER BY id) AS rn
FROM YourTable
GROUP BY id;


Visualising the approach

Let's approach DISTINCT and Row_Number() with an easily digestible, train ride visualization:

Visualize a train with carriages labeled with cities (values):

[ NYC 🚂 | NYC 🚃 | SF 🚃 | LA 🚃 | SF 🚃 | NYC 🚃 ]

Here's DISTINCT in action, picking out unique city destinations:

Unique Destinations: [🗽 NYC , 🌉 SF , 🌴 LA ]

Adding Row_Number() is similar to giving each distinct destination an identifier:

Seating Arrangement: [🗽1, 🌉2, 🌴3]

In SQL:

SELECT DISTINCT city, 
       ROW_NUMBER() OVER (ORDER BY city) as SeatNumber
FROM TrainCarriages;


The result is a numbered list representation:

Result: 
🗽1 (NYC), 
🌉2 (SF), 
🌴3 (LA)

Miscellaneous concepts

Evaluating DISTINCT's necessity

Before using DISTINCT, analyze whether it's necessary. Don't let your query do extra sit-ups if your field is already unique:

-- Sometimes DISTINCT is that extra slice of cake, you don't really need! 🍰
SELECT DISTINCT id
FROM YourTable;


Ascertain function-keyword interaction

Always verify your function and keyword combinations. Remember, with great power comes great responsibility!

-- Dang it! DISTINCT and ROW_NUMBER thought they were friends. Not in this context! 😥
SELECT DISTINCT id, ROW_NUMBER() OVER (ORDER BY id) AS rn
FROM YourTable;


Here, it's possible that using DISTINCT with the id field is unnecessary if the id is unique in your dataset.

Citing resources

1. ROW_NUMBER() in MySQL - Stack Overflow — discussion on using ROW_NUMBER() for distinct values in SQL queries.
2. ROW_NUMBER (Transact-SQL) - SQL Server | Microsoft Learn — the official Microsoft documentation for the ROW_NUMBER() function.
3. SQL Server: Retrieve Top X Rows from a Table for Each Group — Pinal Dave's expertise on using ROW_NUMBER() over PARTITION.
4. SQL Sentry | SolarWinds — lessons on performance tuning SQL queries.
5. - CodeProject — a tutorial on de-duplicating data with ROW_NUMBER().