When you first start writing SQL, pulling data from a database feels like opening a firehose. A basic SELECT * FROM patients; returns every single row in your table. But in real-world data analysis, you rarely want everything at once. You need to target specific insights—such as finding uninsured patients, tracking down records with partial email strings, or filtering by age ranges.

The secret to narrowing down your data lies in the WHERE clause and its rich toolkit of operators. In this guide, we will break down how to use comparison, logical, range, list, NULL, and pattern-matching operators using a practical healthcare dataset.

1. Comparison Operators (=, >, <)

At the core of data filtering are basic comparison operators.…