For those looking at a career in data management, mastering SQL is essential, and subqueries can take one's skills to the next level. SQL, a fundamental tool for managing and manipulating data in relational databases, has become an indispensable skill for businesses relying on data-driven decision-making.

The demand for SQL skills is on the rise, and subqueries are one of the powerful features that can help one extract more complex results. But what exactly are subqueries, and how can they help make SQL queries more efficient and powerful?

This article explores the ins and outs of subqueries in SQL, providing expert insights and tips valuable to anyone looking to become a subquery pro.

Understanding SQL Subqueries

SQL subqueries are powerful tools for data management and manipulation in relational databases. They allow for nested queries within a larger query, enabling the use of the results of one query as input for another.

Subqueries can be utilized in several sections of a SQL statement, including the SELECT, FROM, WHERE, and HAVING clauses. Subqueries can nest queries and compute values that may not be directly available in tables. They can thus greatly enhance the efficiency and power of SQL queries.

For ease of understanding, here's are two hypothetical tables called "orders" and "customers" respectively. This table will illustrate the different ways in which subqueries can be used in SQL.

Using Subqueries in the SELECT Clause

Subqueries can be used in the SELECT clause to compute values that are not directly available in the tables. For example, the following query uses a subquery to compute the average total_amount for all orders:

Now, if someone, for instance, wants to compute the average total_amount for orders placed by customers from the USA, they can use a subquery in the SELECT clause to achieve this:

The subquery retrieves all the customer IDs for customers in the USA, and the main query then uses these IDs to retrieve all orders placed by these customers. The subquery is executed for each row in the "orders" table.

Subqueries in the FROM Clause

Subqueries can also be used in the FROM clause to create temporary tables that are used in the main query. For example, the following query creates a temporary table that counts the number of orders placed by each customer:

Here, the subquery computes the number of orders placed by each customer and creates a temporary table called "order_counts". The main query then retrieves all rows from this temporary table.

Subqueries in the WHERE Clause

Subqueries can also be used in the WHERE clause to filter results based on the results of another query. For example, the following query retrieves all orders placed by customers from the USA:

Here, it retrieves all the customer IDs for customers in the USA, and the main query then uses these IDs to retrieve all orders placed by these customers.

Subqueries with Correlated Queries

Correlated subqueries are subqueries that reference a column from the outer query. This allows for more complex queries that depend on data from both the inner and outer queries. For example, suppose a person wants to find all orders with a total_amount greater than the average total_amount for the customer who placed the order. They can use a correlated subquery to achieve this:

Here, the subquery computes the average total_amount for each customer and checks if the total_amount of the current order (o1) is greater than the computed average total_amount. The subquery is executed for each row in the "orders" table.

Subqueries with EXISTS and NOT EXISTS

The EXISTS operator in SQL is used to check if a subquery returns any rows. This can be useful for filtering data or checking if a particular record exists. For example, suppose a business wants to find all customers who have placed at least one order. They can use the EXISTS operator to achieve this:

Here, the subquery checks if there are any orders placed by the current customer (c) and returns true if no rows are found.

Using Subqueries to Update and Delete Data

Subqueries can also be used to update or delete data in tables. For example, the following query updates the total_amount for all orders placed by customers from the USA by adding a 10% discount:

Output:

The query does not return any output, but it updates the total_amount for all orders placed by customers from the USA.

Similarly, subqueries can also be used to delete data from tables. For example, the following query deletes all orders that have a total_amount less than 50:

Conclusion

With the ability to nest queries and compute values that may not be directly available in tables, subqueries can greatly enhance the efficiency and power of SQL queries. Subqueries can be used to perform complex calculations, filter data, create temporary tables, and update and delete data. These capabilities can be especially useful in large databases where performance optimization is crucial.

To maximize the benefits of SQL subqueries, it is essential to continue learning and exploring their advanced concepts. Cogent University offers a variety of courses and resources to help developers deepen their SQL knowledge and skills. So, anyone wanting to become a proficient SQL developer, must check out the Cogent website and explore more articles on this topic.

‍