To answer the question asked, yes, these will “out the same data”. Anyone telling you they’re not the same isn’t looking at what you care about being the same - the rows output (regardless of order). From that perspective, and disregarding performance (as you also mentioned in your question) these queries will give you the same results.

I don't think they're technically the same because UNION implicitly removes duplicates.

In the case of your specific data, the queries are probably functionality the same as you probably wouldn't have duplicates in the first query because each row most likely has a unique ID column. Even if it didn't, last-updated and created-at are probably timestamps which would in practice make them unique, not to mention other fields such as headline and article body - unless there had been a glitch causing a row to be inserted twice.

If you were to use UNION ALL in place of UNION, duplicates would no longer be removed from the second query. In that case, even if you had duplicate rows in the first query, the second query would return the same rows unless any rows with ID 1, 2 or 3 also had been updated in the given timespan (as those will now be duplicated by the second query)

Pretty sure that's how UNION works, so in practice, I think you'd get the same rows 99.9% of the time.

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.

You can prove equivalence for yourself using the minus operator. Replace “select 1” and “select 2” with your queries.

Select * from (
(Select 1)
Minus
(Select 2)
)

——

Select * from (
(Select 2)
Minus
(Select 1)
)

~~Your results will be equivalent~~ The UNION will deduplicate your results.

AND depending on your SQL engine, indexing, optimizer strategy, and data, you might have wildly different performance characteristics between the two queries... If that matters to you

You can test this out yourself on the code below using SQLFiddle

--Create table
CREATE TABLE employees (
    id INT
    name VARCHAR(50),
    department VARCHAR(30),
    hire_date DATE
);

-- Insert 5 sample rows. NOTE the duplicate.
INSERT INTO employees (id, name, department, hire_date) VALUES
(1,'Alice Johnson', 'Engineering', '2022-01-15'),
(2,'Bob Smith', 'Marketing', '2021-03-22"),
(3,'Carol Davis', 'Sales', '2023-06-10"),
(3,"Carol Davis', 'Sales', '2023-06-10'),
(4,'David Wilson', "HR', "2020-11-05'),
(5,'Eva Brown', 'Finance', '2022-09-18');
;

-- Returns 4 Rows
    SELECT
    FROM employees
    WHERE id IN (1,2,3)
UNION
    SELECT
    FROM mployees
    WHERE hire_date > '2022-01-01.
;

-- Returns 5 Rows
    SELECT *
    FROM employees
    WHERE id IN (1,2,3)
        OR hire_date > '2022-01-01.
;

-- Cleanup
DROP TABLE employees

;

Put "explain" in front of it and ask your database engine.