A subquery in SQL serves the purpose of retrieving data that will be used as a condition in the main query, thereby further refining the data to be retrieved.

More specifically, a subquery—also referred to as a nested query or inner query—is a query that is embedded within the ‘WHERE’ or ‘HAVING’ clause of another SQL query. Its primary function is to return data that acts as a condition for the main query, helping to narrow down the results. Subqueries can be utilized in conjunction with various SQL statements, including ‘SELECT’, ‘INSERT’, ‘UPDATE’, and ‘DELETE’, as well as operators like $=$, $<$, $>$, $\geq$, $\leq$, $IN$, and $BETWEEN$, among others.

Subqueries can return either individual values or a collection of records. Depending on your specific requirements, you can tailor your subquery accordingly. For instance, if you need to find a single value—such as the maximum price of a product—your subquery will be designed to return that single value. Conversely, if you want to identify all products priced above the average, your subquery will yield a list of records.

There are two main types of subqueries in SQL: correlated and uncorrelated. An uncorrelated subquery executes once for the entire query and provides a value or set of values that the outer query can utilize. In contrast, a correlated subquery executes once for each row processed by the outer query and relies on data from that outer query, making its execution dependent on the values from the outer query.

Subqueries can also be nested within other subqueries, with a maximum depth of 32 levels. However, it’s important to note that increasing the levels of nesting can lead to more complex SQL statements, making them harder to read and debug.

In summary, subqueries are a powerful feature of SQL that enable complex data manipulations within a single query. They can simplify your SQL code, enhancing both clarity and maintainability. Nevertheless, it is essential to use them judiciously, as excessive complexity and potential performance impacts can arise from their misuse.