Learn Introduction to Subqueries | Subqueries in E-commerce Analytics

Subqueries are a key feature in SQL that allow you to nest one query inside another. In e-commerce analytics, you often need to answer questions that require looking up related data from multiple tables. A subquery, sometimes called an inner query or nested query, is written inside parentheses and can be placed in various parts of a SQL statement such as the SELECT, FROM, or WHERE clauses.

The basic syntax of a subquery is as follows:

SELECT column1
FROM table1
WHERE column2 IN (
    SELECT column2
    FROM table2
    WHERE condition
);

You use a subquery when you need to filter or compare data based on the result of another query. For example, suppose you want to find all customers who have placed at least one order. You can use a subquery to select customer IDs from the orders table and then use that list to filter the customers table. This approach is especially useful when you need to perform multi-step logic or when a simple join does not directly answer your question.

Subqueries differ from joins in how they approach data retrieval. A join combines columns from two or more tables into a single result set, typically matching rows based on a related column. A subquery, on the other hand, runs a separate query to produce a value or set of values, which is then used by the outer query to filter or compute results. In e-commerce scenarios, subqueries are handy for tasks like identifying customers with orders, finding products never purchased, or filtering based on aggregated data.


              123456
            
SELECT name
FROM customers
WHERE customer_id IN (
    SELECT customer_id
    FROM orders
);

1. Which of the following best describes a subquery in SQL?

2. How does a subquery differ from a join?

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Section 1. Chapter 1

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