These three topics are usually taught as separate chapters in separate books, but in practice they're layers of the same idea, applied at three different altitudes. ACID describes what a single database transaction promises you. SOLID describes what a well-structured class promises the rest of a codebase. Design patterns are the reusable shapes people build, over and over, to actually deliver on both of those promises without reinventing them from scratch every time. This is a full reference, not a quick refresher — work through it in order, or jump to the section you need from the sidebar. All object-oriented examples use C#, since it's the language most interview prep and most enterprise backend code in this space is actually written in; the ideas transfer directly to Java, TypeScript, or any other statically-typed OO language.