Select rows with same id but different value in another column

sql

join

distinct

performance

byAnton Shumikhin·Oct 7, 2024

Quickly identify unique id-value pairs where an id corresponds to multiple different values with the following subquery:

SELECT *
FROM myTable
WHERE id IN (
    SELECT id
    FROM myTable
    GROUP BY id
    HAVING COUNT(DISTINCT value) > 1
);

This slick operation earmarks rows where an id has more than one unique value.

Layman's terms

Imagine running a library. Each customer (id) borrows multiple books (value). You want to find customers who have borrowed more than one distinct book. In other words, those bibliophiles enjoying a diverse reading palette!

Customer_ID	Borrowed_books
1	Crime
1	SciFi
2	Romance
2	Romance

You're after those genre-hopping readers:

Customer 1: Crime ↔ SciFi (same customer, different books)

SELECT DISTINCT a.ID
FROM library a, library b
WHERE a.ID = b.ID AND (a.Borrowed_books != b.Borrowed_books);

The SQL query becomes your librarian, picking out only those customers exploring a variety of literature.

Let's get technical

The INNER JOIN Approach

Combine (or "Join") the table with itself to find rows with the same id but different value:

SELECT DISTINCT a.*
FROM myTable a
INNER JOIN myTable b ON a.id = b.id AND a.value <> b.value;

Here, the <> operator is your friend, ensuring you're comparing different value entries for the same id.

Window Functions to the Rescue

Window functions can potentially increase efficiency in larger tables:

SELECT DISTINCT id, value
FROM (
    SELECT id, value,
    COUNT(DISTINCT value) OVER (PARTITION BY id) as val_count
    FROM myTable
) subquery
WHERE val_count > 1;

Count like a Pirate (Arghh... with a twist!)

COUNT(DISTINCT ...) can sometimes give performance challenges, so count differently:

SELECT id
FROM myTable
GROUP BY id
HAVING MIN(value) <> MAX(value);

Here the MIN= MAX for id with only one value. Anything else... we nab!

Handling Anomalies

Null Values' Shenanigans

SELECT DISTINCT a.*
FROM myTable a
INNER JOIN myTable b ON a.id = b.id AND (a.value <> b.value OR a.value IS NULL AND b.value IS NOT NULL);

This trick ensures id pairs with one null value aren't party-poopers and stick around for the fun.

Performance Jitters

To prevent slow operation, ditch distinct operations and go incognito:

SELECT id
FROM myTable
GROUP BY id
HAVING SUM(CASE WHEN value = 'specific_value' THEN 1 ELSE 0 END) = 0;

Razzle-dazzle 'em! Evade COUNT(DISTINCT ...) by identifying ids without a 'specific_value'.

A Cut above the Rest

Battling Large Datasets

When dealing with large datasets, strategy counts:

Use indexed fields for id and value to speed up queries.
Adopt batch processing and temporary tables or table variables to chunk workload.

Deciphering Data Model

Design database to ensure uniqueness constraints and avoid redundant data.
Maintain referential integrity between tables with foreign keys.

Wrestling with Duplicates

Remove duplicates before entering the wrestling ring of comparisons:

WITH CTE_Deduped AS (
    SELECT DISTINCT id, value
    FROM myTable
)
SELECT *
FROM CTE_Deduped
WHERE id IN (
    SELECT id
    FROM CTE_Deduped
    GROUP BY id
    HAVING COUNT(*) > 1
);