Mysql: Select DISTINCT / UNIQUE, but return all columns?

Diving into GROUP BY - Managing distinct rows

Here we peek into the world of GROUP BY, picking up unique entries and roping in all columns.

SELECT columnA, MAX(columnB), MAX(columnC), ...
FROM mytable
GROUP BY columnA;


The trick is to envelop other columns within aggregate functions like MAX() or MIN(). But, beware! This method might give you mixed column values not reflecting the complete row data. Hence, handle with care and use only on datasets where approximations float your boat.

ROW_NUMBER() - The precise scalpel of SQL

A surgeon needs precision, and so does a programmer. Use window functions with the usual suspects (GROUP BY or DISTINCT) to cut through the data accurately:

SELECT mt.*
FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY columnA ORDER BY columnB) as rn
  FROM mytable
) mt
WHERE mt.rn = 1;
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ROW_NUMBER() lets you handpick the row representing each distinct value, keeping you compliant with standard SQL functionality. The power of choice — it's all yours.

Wrestling with complex challenges

The "group" of uncertainty

When juggling multiple columns with GROUP BY, you might experience unpredictable behavior. Define clear criteria for grouping and don't shy away from using aggregate functions to drive away ambiguity.

The duplicate offside

Duplicates in distinct columns can be a game-changer affecting your outputs. Business logic and distinct strategies must align to tackle such corner cases. No hidden surprises here!

The WHERE secret weapon

Exploit the powers of WHERE clause to trim the data to meet specific criteria. Using pattern filters like LIKE, coupled with GROUP BY or DISTINCT, gifts you clear and meaningful results.

Mastery in unique row fetching

Breaking limits with subqueries

Sometimes, reaching the finish line requires a few hurdles along the way. Utilizing an ORDER BY combined with LIMIT 1 within your subquery can guide you to the correct row:

SELECT mt.*
FROM mytable mt
WHERE mt.ID = (
    SELECT sub.ID
    FROM mytable sub
    WHERE sub.columnA = mt.columnA
    ORDER BY sub.columnA, sub.someDate DESC
   -- "I'm the most recent, Pick me!" 😉
    LIMIT 1
);
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Building the performance ladder

To boost performance, use indexes on the fields you use for JOINs or inside the PARTITION BY clause. Think of it as steroids for your queries, but legal!