To efficiently learn database concepts, this work adopts association rules to provide diagnostic feedback for drawing an Entity-Relationship Diagram (ERD). Using association rules and Asynchronous JavaScript and XML (AJAX) techniques, this work implements a novel Web-based Timely Diagnosis System (WTDS), which provides timely diagnostic feedback whenever a learner encounters hurdles when learning database concepts. The WTDS provides crucial hints that help students rectify their misconceptions at each step to prevent prospective mistakes. To identify the learning effects of various feedback types, this work compares the proposed WTDS with systems providing other feedback types. The evaluation results demonstrate that all systems enhance learning achievement significantly. However, learning achievement of the class using the WTDS was significantly better than that of other classes, and the learning achievements of all cluster levels in the class using the WTDS were enhanced significantly. Finally, questionnaires and interviews were used to acquire student attitudes about the proposed system.