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Introduction to Modern Databases

Conditional Logic in SQL

Explore how to use SQL conditional logic tools such as the CASE statement and IF() function to create dynamic categorizations in query results. Understand ways to handle NULL values using COALESCE() and NULLIF() functions to improve data accuracy and reporting. This lesson equips you with practical skills for writing flexible, readable SQL queries that categorize and manage data intelligently.

We'll cover the following...

Imagine we’re running our online store and want to create a special “Top Sellers” report.

We need to categorize products based on their monthly sales: any product selling more than 15 units is a ‘Top Seller’, those selling between 5 and 15 are ‘Regular Sellers’, and anything less is a ‘Slow Mover’. How can we add these custom labels directly into our SQL query results?

Simple filtering with WHERE won’t work because we want to see all products, just with this new descriptive label.

This is where conditional logic comes in - it lets us embed if-then-else style decision-making right into our queries. By the end of this lesson, we’ll be able to write queries that can handle exactly this kind of dynamic categorization and much more.

Specifically, we will learn how to:

  • Use the CASE statement to apply complex if-then-else logic.

  • Use the IF() function for simple, binary conditions.

Handle NULL values gracefully using COALESCE() and NULLIF().

The CASE statement

The CASE statement is one of the most powerful tools for conditional logic in SQL. It allows us to apply if-then-else logic within a query to create dynamic columns, categorize data, and make our results much more readable. Instead of just retrieving data as it's stored, CASE lets us transform it on the fly based on conditions we define.

A CASE statement evaluates a ...