It is important to learn about both inner and outer joins and they have different matching behavior.
I would start out with our article Introduction to Database Joins.
This first article introduces the concept of joining tables. The focus is going to be more on the type of joins, not necessarily their syntax.
The later articles focus on the various types of joins. Through the narrative and examples you’ll become very comfortable with each one.
Some of our more popular articles include:
Recommended Database Join Articles
All the examples for this lesson are based on Microsoft SQL Server Management Studio and the sample databases AdventureWorks and WideWorldImporters. You can get started using these free tools using my guide Getting Started Using SQL Server