Correlated subquery is used for row-by-row processing. With a normal nested subquery, the inner SELECT query runs
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Correlated subquery is used for row-by-row processing. With a normal nested subquery, the inner SELECT query runs once and executes first. The returning values will be used by the main query. A correlated subquery, however, executes once for every row of the outer query. In other words, the inner query is driven by the outer query
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