Aggregate Functions, GROUP BY, HAVING, and Window Functions
Use SQL Server for reporting, analytics, rankings, summaries, and advanced calculations over business data.
Inside this chapter
- Why Aggregation Matters
- Core Aggregates
- Filtering Grouped Results
- Window Functions for Advanced Queries
Series navigation
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Why Aggregation Matters
Applications and managers often need totals, counts, averages, and grouped summaries rather than raw detail rows. SQL Server can compute these efficiently close to the data, which reduces application complexity and improves clarity.
Core Aggregates
SELECT
OrderStatus,
COUNT(*) AS TotalOrders
FROM dbo.Orders
GROUP BY OrderStatus;Functions such as COUNT, SUM, AVG, MIN, and MAX are foundational for reporting work.
Filtering Grouped Results
SELECT
CustomerId,
COUNT(*) AS TotalOrders
FROM dbo.Orders
GROUP BY CustomerId
HAVING COUNT(*) >= 5;HAVING filters aggregated results, while WHERE filters individual rows before grouping. Understanding that difference prevents many beginner errors.
Window Functions for Advanced Queries
SELECT
OrderId,
CustomerId,
OrderDate,
ROW_NUMBER() OVER (
PARTITION BY CustomerId
ORDER BY OrderDate DESC
) AS CustomerOrderRank
FROM dbo.Orders;Window functions are extremely useful for ranking, running totals, latest-per-group logic, and advanced reporting. They are essential for intermediate and advanced SQL Server work.