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623. JOIN Selectivity

Understand how the uniqueness and distribution of JOIN values affect query performance.

📘 Lesson 623 of 728 ⏱ 14 min 🎯 Advanced

Selectivity describes how effectively a condition reduces the number of matching rows. Highly selective conditions usually identify fewer rows.

Understanding selectivity helps explain why some JOINs and filters perform better than others.

A JOIN using a unique CustomerID generally has different behavior from a JOIN using a common Country value.

SQL 
SELECT
    c.CustomerID,
    c.CustomerName,
    o.OrderID
FROM Customers c
INNER JOIN Orders o
    ON c.CustomerID = o.CustomerID
WHERE c.CustomerID = 1001;

Used in database optimization, index design, query tuning, and performance troubleshooting.

  • Selective conditions reduce matching rows.
  • Unique keys are generally highly selective.
  • Low-selectivity columns may match many rows.
  • Data distribution influences optimizer decisions.
  • Indexes are more useful for some selective workloads than others.
What is selectivity in SQL?

Selectivity describes how much a condition narrows the possible rows.

Why does selectivity matter for JOIN performance?

Highly selective conditions can reduce the amount of data the database needs to process.

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