A subquery that follows the keywords ALL, ANY, or SOME is called a quantified comparison. The optimizer converts these subqueries to joins if:
The main query does not contain a GROUP BY clause, and is not an aggregate query, or the subquery returns exactly one value.
The subquery does not contain a GROUP BY clause.
The subquery does not contain the keyword DISTINCT.
The subquery is not a UNION query.
The subquery is not an aggregate query.
The conjunct 'expression comparison-operator { ANY | SOME } ( subquery-expression )' must not be negated.
The conjunct 'expression comparison-operator ALL ( subquery-expression )' must be negated.
The first four of these conditions are relatively straightforward.
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