Subqueries in the SELECT Clause

The Inline Calculation

For each order, I needed to show the customer's total historical spend. A JOIN would duplicate rows. A SELECT subquery was cleaner.

SELECT

o.order_id,

o.order_amount,

(SELECT SUM(amount) FROM orders WHERE customer_id = o.customer_id) AS lifetime_spend

FROM orders o;

Adding a single, calculated value per row.

When a JOIN would be overly complex or cause row duplication.

This is essentially a correlated subquery—it runs once per row. For large tables, consider using a CTE or Window Function for better performance.

*Day 53: The Power of ANY and ALL.*