Sql distinct for 2 fields in a database

sql

distinct

group-by

indexing

byAnton Shumikhin·Sep 7, 2024

This SQL command selects distinct combinations of two columns:

-- This is the one-liner that you are looking for
SELECT DISTINCT columnA, columnB FROM yourTable;

Just replace columnA, columnB, and yourTable with your actual fields and table name to retrieve unique pairings from the specified columns.

Drill Down - Retrieve and Count Unique Pairs

To fetch distinct combinations and count the frequency of these pairings in your table, use:

-- Count those lovely pairs
SELECT c1, c2, COUNT(*) as pairing_occurrences
FROM yourTable
GROUP BY c1, c2;

GROUP BY bundles identical data, while COUNT(*) in SQL allows us to count the appearances of each unique value pair from c1 and c2.

Performance Tweaking - Speed It Up!

Large data sets can make DISTINCT and counting a performance nightmare. To speed things up, consider indexing your columns:

-- Speedy Gonzales style indexing
CREATE INDEX runway ON yourTable(c1, c2);

By creating indexes, the query planner greases its wheels and zoom-zoom! Your query execution time very often goes down exponentially.

The Plot-twists - Null Values, and How to Handle Them

Remember, DISTINCT can behave unpredictably with null values. Avoid trouble with this special case:

-- "Null and Void" operations, coming up!
SELECT DISTINCT c1, c2 FROM yourTable WHERE c1 IS NOT NULL AND c2 IS NOT NULL;

By excluding null values, the results are more robust as comparing null values tends to take us down the rabbit hole.

Other Handy SQL Tools - Group and Window wisely

Aside from DISTINCT, SQL also provides GROUP BY and window functions such as ROW_NUMBER(). These offer more nuanced control over data grouping and selection:

-- "Window of Opportunity" abound!
SELECT c1, c2,
       ROW_NUMBER() OVER (PARTITION BY c1, c2 ORDER BY extraColumn) as rn
FROM yourTable
WHERE rn = 1;