Join Variants

In the world of data analysis, we often need to compare records, link them together in creative ways, or identify things that do not have a match.

Imagine we are building a product alert system: we want to notify managers if any products have not been restocked for over a month and have no suppliers listed. A simple join will not do; we need more nuanced ways to connect and contrast data.

That is where Join variants come in. In this lesson, we’ll explore advanced join patterns such as self-joins, anti-joins, and non-equi joins.

These techniques expand our problem-solving toolkit when working with complex data relationships.

By the end of this lesson, we’ll be able to:

• Identify situations where advanced joins are necessary.

• Use self-joins to compare rows within the same table.

• Use anti-joins to find non-matching rows between tables.

• Apply non-equi joins to compare rows using inequality conditions.

Pattern overview

Category:

• Comparison Patterns

Intent:

To connect rows in non-standard ways by comparing values within the same table, excluding matches, or using inequality conditions.

Motivation:

Standard INNER JOIN and LEFT JOIN operations are powerful, but they don’t always get the job done. When we need to find unmatched records, relate rows within the same table, or compare ranges, we turn to join variants.

Also known as:

• Self join: Reflexive join

• Anti join: Exclusion join

• Non-equi join: Inequality join

Structure

1. Self join: Used to relate a table to itself. This is helpful when rows are connected by a parent-child, referral, or comparison relationship.

Given the following structure of the Customers table: