"A query that ran perfectly during development suddenly slows down as data accumulates." At such times, the mark of a truly skilled developer is identifying the root cause based on solid evidence rather than guesswork, and optimizing a 30-second query down to 0.1 seconds. Master the core principles of database performance—from execution plans, indexes, and optimizers to joins and sorting—using millions of rows of real-world data. Elevate your skills and become a top-tier developer capable of independently analyzing and resolving slow queries in production environments.

Solve the challenges you inevitably face in real-world design—such as common codes, category hierarchies, history tracking, soft deletes, statistics, dynamic design, and JSON utilization—with 9 core patterns. This is not just simple theory; it contains practical know-how and selection criteria for various scenarios, proven across numerous projects. Now, become a true design expert who can confidently handle any requirement that comes your way.

You will systematically learn database design through three stages—conceptual, logical, and physical—and strengthen your skills through a practical shopping mall project. Gain real-world know-how through modern design techniques and the latest trends.

Learn the fundamentals of SQL and databases essential for practical work. Not only developers, but also planners and data analysts can learn SQL in depth.

Not only developers, but also planners and data analysts learn databases step-by-step, starting from the basics of SQL.

Learn the essential modern Java technologies of Lambda, Stream, and functional programming with Java, from the basics to the practical level, in depth.

Learn I/O, networking, reflection, and annotations in depth, from the basics to the practical level. Build a web application server (WAS) directly in Java.

This course provides an in-depth study of multithreading and concurrency, from basic principles to practical applications.

Learn Java generics and collection frameworks in depth with a focus on practical applications. Also learn the basics of data structures.

This book provides an in-depth study of various intermediate-level Java features necessary for practical work, using example code.

This is a video of the meetup that took place at 7pm on January 31st.

Learn practical core Java object-oriented concepts easily via example code.