What is the role of a subquery in SQL?

A subquery in SQL is used to retrieve data that will be used in the main query as a condition to further restrict the data to be retrieved.

In more detail, a subquery, also known as a nested query or inner query, is a query embedded within the WHERE or HAVING clause of another SQL query. It is used to return data that will be used in the main query as a condition to further restrict the data that is to be retrieved. Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc.

Subqueries can return individual values or a list of records; depending on how much data you need to retrieve, you can adjust your subquery accordingly. For example, if you only need to find a single value (like the maximum price of a product), your subquery will return a single value. But if you need to find all products that are above the average price, your subquery will return a list of records.

There are two types of subqueries in SQL: correlated and uncorrelated. An uncorrelated subquery runs once for the entire query and returns a value or set of values. On the other hand, a correlated subquery runs once for each row processed by the outer query. It relies on data from the outer query and its execution is dependent on the outer query.

Subqueries can be nested inside other subqueries, up to a limit of 32 levels. However, the more levels of nesting, the more complex the SQL statement becomes, which can make it harder to read and debug.

In conclusion, subqueries are a powerful feature of SQL that allow you to perform complex data manipulations with a single query. They can simplify your SQL code and make it easier to understand and maintain. However, they can also increase the complexity of your queries and potentially impact performance, so they should be used judiciously.