Professional SQL That Will Make Your Queries Shine: INTERSECT vs. INNER JOIN

When working with databases, it’s crucial to know the best methods to query data effectively. In SQL, two powerful operations that allow you to compare sets of data are the INTERSECT operator and the INNER JOIN clause. Both commands serve the purpose of identifying commonalities between datasets, but they do so in subtly different ways that can affect the output and performance of your queries.

What is the INTERSECT Operator?

The INTERSECT operator is used to return a distinct intersection of two sets, which means it will only return the rows that are present in both query result sets. Here are the key features of INTERSECT:

• Distinct Results: Automatically removes duplicates.
• NULL Value Comparisons Are Respected: Includes NULL comparisons, treating them distinctly.
• Structural Requirements: Requires matching column order and data types, though column names can differ.

What about INNER JOIN?

INNER JOIN, on the other hand, is commonly used for merging rows from two or more tables based on a related column between them. Here’s what you should know about INNER JOIN:

• Comprehensive Output: Returns rows with columns from both tables.
• Handling NULL Values: Does not respect NULL values; such comparisons need explicit handling.
• Duplicates: Can return duplicates, depending on the number of matches—requiring adjustments in the SELECT clause to manage them.

Handling NULLs and Duplicates

When using INNER JOIN, you may encounter issues with NULL values and duplicates. Here’s how you can manage them:

• NULL Values: Use the COALESCE function to replace NULL with a non-null default value, allowing the row to be included in the results.
• Duplicates: To avoid duplicates, you can explicitly specify the DISTINCT keyword in your SELECT clause.

Which Should You Use?

Choosing between INTERSECT and INNER JOIN depends on your specific needs:

• Use INTERSECT for straightforward comparisons that require distinct results. Learn more here.
• Opt for INNER JOIN when you need more flexibility in selecting and displaying columns or when handling complex relationships. Learn more here.

For a deeper dive into the nuances of these SQL operations, I’ve included a detailed PDF in this post that elaborates further on these differences and how to effectively use each operation.