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If my understanding is correct, a semi-join is a way of selecting records from one dataset that have related (via a common attribute) records in another set. So where does the "semi" come from?

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"from one dataset"--of two. 1/2. "semi".

The paper that introduced the term is Using Semi-Joins to Solve Relational Queries by Bernstein and Chiu (1981).

this operator, which we call the semi-join and denote by ⋉,[...] takes the join of two relations, R and S, and then projects back out on the domains [attributes/columns] of relation R.

(Typically now we say either left (theirs) or right semi-join.)

They don't give a rationale. But the idea is that (informally) the semi-join is (the projection on) the R or S "half" of the join.

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  • Great answer - thanks for the extra info!
    – Johntron
    Commented Jan 26, 2023 at 14:33

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