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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[โ€“] RobertTableaux@programming.dev 8 points 1 week ago (1 children)

The UNION removing any dups here is what makes them the same - the top query would never have duplicates as written.

[โ€“] rollin@piefed.social 2 points 1 week ago

Ah but that's true ONLY IF the table doesn't itself contain duplicates. Quick example:

CREATE TEMPORARY TABLE animal (species VARCHAR(255) NOT NULL, colour VARCHAR(255) NOT NULL);
INSERT INTO animal VALUES ('monkey', 'green'), ('rabbit', 'orange'), ('elephant', 'pink'),('monkey','blue'),('rabbit','orange'),('monkey','green'),('monkey','green');

SELECT * FROM animal WHERE species = 'monkey' OR colour = 'green';
+---------+--------+
| species | colour |
+---------+--------+
| monkey  | green  |
| monkey  | blue   |
| monkey  | green  |
| monkey  | green  |
+---------+--------+
SELECT * FROM animal WHERE species = 'monkey' UNION SELECT * FROM animal WHERE colour = 'green';
+---------+--------+
| species | colour |
+---------+--------+
| monkey  | green  |
| monkey  | blue   |
+---------+--------+

So we could change the query to use UNION ALL, which does include duplicates. In that case, the returned rows are the same ONLY IF the rows returned by the left side of the UNION do not overlap those returned by the right side, otherwise it will return more rows.

SELECT * FROM animal WHERE species = 'monkey' UNION ALL SELECT * FROM animal WHERE colour = 'green';
+---------+--------+
| species | colour |
+---------+--------+
| monkey  | green  |
| monkey  | blue   |
| monkey  | green  |
| monkey  | green  |
| monkey  | green  |
| monkey  | green  |
| monkey  | green  |
+---------+--------+

For completeness, here's an example where the two queries in the UNION do not return any of the same rows:

SELECT * FROM animal WHERE species = 'monkey' OR colour = 'orange';
+---------+--------+
| species | colour |
+---------+--------+
| monkey  | green  |
| rabbit  | orange |
| monkey  | blue   |
| rabbit  | orange |
| monkey  | green  |
| monkey  | green  |
+---------+--------+
SELECT * FROM animal WHERE species = 'monkey' UNION ALL SELECT * FROM animal WHERE colour = 'orange';
+---------+--------+
| species | colour |
+---------+--------+
| monkey  | green  |
| monkey  | blue   |
| monkey  | green  |
| monkey  | green  |
| rabbit  | orange |
| rabbit  | orange |
+---------+--------+