Regular sub query and a correlated sub query

Question

Describe the difference between a regular sub query and a correlated sub query?

Answer 1

A correlated sub query seems deceptively similar to a regular sub query. The dissimilarity is that a regular sub query can be processed from the bottom up. In a regular sub query, outcomes from the lowest query can be examined and employed to evaluate the upper-level query. In contrary, in a correlated sub query, the processing is nested; which is, a row from an upper query statement is employed in comparison with rows in a lower level query. The key distinction of a correlated sub query is that the lower-level select statements employ columns from upper-level statements.