We'll walk through the operators you actually need as a beginner—comparison, logical, range and list, NULL handling, and pattern matching—all in PostgreSQL, with queries I actually ran against a patients table.

Comparison Operators

Comparison operators are the ones you'll use every single day. They return a boolean for every row, and WHERE keeps only the rows where the result is true . The core set is = , <> (or != ), < , > , <= , and >= .

Here's a query for a patients table:

SELECT patient_id , patient_name , bill_amount FROM patients WHERE bill_amount > 10000 ;

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Simple enough. The trouble starts when you combine conditions.

Logical Operators: AND & OR

AND requires both sides to be true. OR requires at least one. NOT negates a condition.…