A subquery can produce a temporary result set that can be joined with another table. This is useful when data must be filtered or aggregated before it is combined.
Preprocessing data inside a subquery can make complex analytical queries easier to understand and control.
Calculate customer order totals first and then join those totals to the Customers table.
SELECT
c.CustomerName,
x.TotalSales
FROM Customers c
INNER JOIN (
SELECT
CustomerID,
SUM(OrderAmount) AS TotalSales
FROM Orders
GROUP BY CustomerID
) x
ON c.CustomerID = x.CustomerID;Used in financial reporting, customer analytics, sales analysis, and complex data transformations.
- A subquery can act as a derived table.
- The derived result can be joined.
- Aggregation can happen before the JOIN.
- Aliases are required for derived tables in many SQL systems.
- This approach can simplify complex queries.
Can a subquery be used in a JOIN?
Yes. A subquery can produce a derived result set that can be joined with another table.
Why aggregate inside a subquery before JOINing?
It can reduce the data being joined and prevent unnecessary row multiplication.