this post was submitted on 10 Aug 2025
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Are these queries equivalent? (lemmy.ca)
submitted 1 week ago* (last edited 1 week ago) by roadrunner_ex@lemmy.ca to c/sql@programming.dev
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Putting aside any opinions on performance, I've been trying to test a notion about whether a couple queries would output the same data (ordering doesn't matter).

SELECT *
FROM articles
WHERE (
  last_updated >= %s
  OR id IN (1, 2, 3)
  )
  AND created_at IS NOT NULL

SELECT *
FROM articles
WHERE last_updated >= %s
  AND created_at IS NOT NULL
UNION
SELECT *
FROM articles
WHERE id IN (1, 2, 3)
  AND created_at IS NOT NULL

I think they're equivalent, but I can't prove it to myself.

Edit: Aye, looking at the replies, I'm becoming aware that I left out a couple key assumptions I've made. Assuming:

a) id is a PRIMARY KEY (or otherwise UNIQUE)

b) I mean equivalent insofar as "the rows returned will contain equivalent data same (though maybe ordered differently)"

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[–] boatswain 2 points 1 week ago (3 children)

UNION is used to append the result of one query to the result of another: https://learn.microsoft.com/en-us/sql/t-sql/language-elements/set-operators-union-transact-sql?view=sql-server-ver17

A UNION concatenates result sets from two queries. But a UNION does not create individual rows from columns gathered from two tables. A JOIN compares columns from two tables, to create result rows composed of columns from two tables.

Your two queries are not equivalent.

[–] Deebster@programming.dev 2 points 1 week ago* (last edited 1 week ago) (2 children)

There aren't any joins in either query (and only one table involved), so that quoted bit of documentation isn't relevant.

Order aside, the two results would be identical In the same way that 2(4 + 3) = 2×4 + 2×3

[–] boatswain 1 points 1 week ago (1 children)

I really didn't think that's correct--though it's been a few years since I did SQL regularly.

SELECT *
FROM articles
WHERE last_updated >= %s
  AND created_at IS NOT NULL
UNION
SELECT *
FROM articles
WHERE id IN (1, 2, 3)
  AND created_at IS NOT NULL

That should give a list of all articles updated after whatever date (regardless of ID), appended to a list of all articles where the ID is 1, 2, or 3 (regardless of when they were last updated). I would expect to see extra articles that only fit one criteria or the other, and also duplicate articles.

I included the join quote because an inner join would be the way to do this, rather than a union--though it would likely be less efficient than just filtering on the required parameters.

If I'm wrong here, I'd love an explanation of why.

[–] Deebster@programming.dev 1 points 1 week ago

It's equivalent because UNION removes duplicates; the behaviour you're describing happens with UNION ALL. Since both queries are article.*, both halves will have the same columns and the dedupe will be successful.

UNION is less efficient because of this deduplication, but it's the default since that's what most people want. If that matters then you'd be correct that a JOIN version will be more efficient (possibly depending on indexes present and sql engine).