Pre-aggregation means summarizing a detailed table to the required grain before JOINing it with another dataset.
Pre-aggregation can prevent inflated totals and reduce the amount of data processed by later JOIN operations.
Calculate total sales per customer before joining the result to the Customers table.
WITH SalesSummary AS (
SELECT
CustomerID,
SUM(OrderAmount) AS TotalSales
FROM Orders
GROUP BY CustomerID
)
SELECT
c.CustomerName,
s.TotalSales
FROM Customers c
LEFT JOIN SalesSummary s
ON c.CustomerID = s.CustomerID;Used in financial reports, customer analytics, data warehouses, and large-scale reporting queries.
- Aggregate detailed data before JOINing when appropriate.
- Pre-aggregation can prevent row multiplication.
- It can reduce intermediate result size.
- The aggregated grain must match the JOIN requirement.
- CTEs and derived tables are useful for pre-aggregation.
What is pre-aggregation?
It means summarizing detailed data to the required level before JOINing it with another dataset.
How can pre-aggregation prevent errors?
It can reduce one-to-many detail records to one row per entity before the JOIN, preventing unwanted row multiplication.