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I am convinced that it is a very basic question, but I still have to ask it, making queries on the Oracle HR schema I notice that the result of these two queries is different:

enter image description here

Expected result: enter image description here

I would like to know why this happens, when using the parentheses it gives me the result I want, while if I do not use the parentheses it gives me another record (Pat Fay) that does not make sense, how should I read this sentence to understand these outputs, thank you very much .

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    Because AND has higher operator precedence than OR. This concept applies generally to almost all languages - precedence matters.
    – SMor
    Dec 18, 2020 at 4:22
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    This order of precedence doesn't ONLY apply to "almost all languages". It is a basic, fundamental mathematical concept that one learns on the first day of Algebra I. In fact, I'd say that if there is a language that does not implement it ("applies generally to almost all languages) then that language is fundamentally flawed.
    – EdStevens
    Dec 18, 2020 at 15:44

1 Answer 1

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it's about order of operations: "and" happens before "or", so

where department_id=20 or department_id=50 and salary >= 8000

is actually the same as

where department_id=20 or ( department_id=50 and salary >= 8000 )

and has a different result than

where ( department_id=20 or department_id=50 ) and salary >= 8000

where the parenthesis force the order of operations to be different and the department ids are evaluated together.

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