A subquery in the SELECT clause of the main query is called a scalar subquery.
This is a single row, single column query, which looks just like a column or function in the SELECT clause. The structure is:
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A correlated subquery is a type of nested subquery that uses columns from the outer query in its WHERE clause.
For example, a query to list employees whose salary is more than their department’s average:
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A subquery is – to put it simply – a query within a query.
What purpose does a subquery serve?
A subquery may be needed when it takes more than a single step to reach the answer.
Suppose we need to find all employees who work in the same department as KING. We need to:
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