Nested queries allow users to embed one SQL query within another query. A nested query contains a subquery, which is typically placed in the WHERE clause. Subqueries can also appear in the FROM or HAVING clauses. The GROUP BY clause allows applying aggregate functions like COUNT, SUM, AVG to each group of rows, with the HAVING clause filtering the groups based on conditions.

1. Nested Queries
One of the most powerful features of SQL is nested
queries
A nested query is a query that has another query
embedded within it the embedded query is called a
subquery.
A subquery typically appears within the WHERE clause of
a query
Subqueries can sometimes appear in the FROM clause
or the HAVING clause

8. GROUP BY and HAVING Clauses
we have applied aggregate operations to all
(qualifying) rows in a relation.
Often we want to apply aggregate operations to
each of a number of groups of rows in a relation,
where the number of groups depends on the relation
instance