Hidden Pitfalls with INNER JOIN and NOT IN Operators

I want to show you how two common operators can deceive you into believing you have constructed a proper SQL statement for your solution. We may want to filter results in a table by using using a IN/NOT IN or a INNER JOIN/LEFT JOIN to another table. There are a couple of hidden dangers here which I will describe.

Imagine we want to evaluate a measurable event and compare it to  another event. This is typical in business. How many people that bought motor oil from Walmart also bought auto parts ? How many of my customers that bought a