PARTITION BY vs. GROUP BY in SQL: What's the Difference?

Understanding the Differences Between PARTITION BY and GROUP BY in SQL

Partitioning and grouping are crucial operations in SQL for data aggregation and processing. While both PARTITION BY and GROUP BY involve dividing and aggregating data, they differ significantly in their functionality and applications.

PARTITION BY: Partitioning for Window Functions

PARTITION BY is primarily used in conjunction with window functions, such as ROW_NUMBER(), which perform calculations based on a defined partition. It divides the data into distinct groups based on specified columns, known as partition keys. Each partition operates independently, allowing window functions to calculate values relative to their respective partitions.

For example, the following query uses PARTITION BY to assign sequential numbers to rows within each customer ID:

SELECT ROW_NUMBER() OVER (PARTITION BY customerId ORDER BY orderId) AS OrderNumberForThisCustomer
FROM Orders;

GROUP BY: Aggregating Data into Groups

GROUP BY, on the other hand, is designed for aggregating data across multiple rows based on common values. It groups rows with matching values in specified columns, referred to as grouping keys. The aggregation function, such as COUNT(*) or SUM(), is then applied to each group.

The following query uses GROUP BY to calculate the total number of orders for each customer:

SELECT customerId, COUNT(*) AS orderCount
FROM Orders
GROUP BY customerId;

Key Differences

The main differences between PARTITION BY and GROUP BY can be summarized as follows:

• Purpose: PARTITION BY partitions data for window functions, while GROUP BY aggregates data into groups.
• Effect on Result: PARTITION BY does not reduce the number of returned rows, while GROUP BY typically reduces the number of rows by grouping and aggregating.
• Window Functions: PARTITION BY is compatible with window functions, enabling calculations within partitions. GROUP BY does not support window functions.
• Flexibility: PARTITION BY allows for flexible partitioning based on multiple columns, while GROUP BY is limited to grouping based on the specified columns.

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