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A subquery comes in two general flavors, one known as a "scalar select"
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which specifically must return exactly one row and one column, and the
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other form which acts as a "derived table" and serves as a source of
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to be placed in the :term:`WHERE clause`, :term:`columns clause`,
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ORDER BY clause or HAVING clause of the enclosing select, whereas the
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derived table form is eligible to be placed in the FROM clause of the