SQL-Tutor is the first constraint-based tutor. The initial conference papers about the system were published in 1998 (Mitrovic 1998a, 1998b, 1998c), with an "IJAIED" paper published in 1999 (Mitrovic and Ohlsson, "International Journal Artificial Intelligence in Education," 10(3-4), 238-256, 1999). We published another "IJAIED" paper in 2003, focussed on the Web-enabled version of the same system (Mitrovic, "Artificial Intelligence in Education," 13(2-4), 173-197, 2003). In this paper, we discuss the reasons for developing the system, our experiences with the early versions, and also provide a history of later projects involving SQL-Tutor.