SQL PARTITION BY

Understanding SQL PARTITION BY

PARTITION BY is a powerful clause used within window functions to divide rows into groups (partitions) for calculations. Unlike GROUP BY which collapses rows, PARTITION BY maintains all rows while performing calculations within each partition.

Basic Syntax

Common PARTITION BY Interview Questions

What's the difference between PARTITION BY and GROUP BY?
How can you calculate percentages within groups using PARTITION BY?
When would you use multiple columns in a PARTITION BY clause?
How can you find the most viewed video for each user using PARTITION BY?

Using PARTITION BY with Different Functions

1. Aggregate Functions with PARTITION BY

Use PARTITION BY with aggregate functions to calculate totals and averages within groups.

2. Ranking Functions with PARTITION BY

Combine PARTITION BY with ranking functions to rank rows within groups.

3. Multiple Partition Columns

Use multiple columns in PARTITION BY for more granular grouping.

Advanced PARTITION BY Examples

Example 1: User Engagement Analysis

Analyze user engagement patterns across different time periods:

Example 2: Content Performance Metrics

Compare video performance against channel averages:

Best Practices for PARTITION BY

1. Choose Partition Columns Carefully

Select partition columns that create meaningful groups for your analysis. Too many partition columns can lead to small, less meaningful groups, while too few may not provide enough granularity.

2. Consider Performance

Large partitions can impact query performance. Consider these tips:

Index columns used in PARTITION BY clauses
Avoid creating too many small partitions
Filter data before applying window functions

3. Combine with ORDER BY Wisely

When using PARTITION BY with ORDER BY, ensure the sort order makes sense for your analysis:

Use ORDER BY for running totals and moving averages
Consider both ASC and DESC ordering based on your needs
Include multiple columns in ORDER BY for consistent results

Interview Master

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