Free Hierarchical SQL Tutorial

IMS is a pure tree structure that the user navigates. This was the basisfor all serious, large-scale DP work, so people assumed that if SQL could nothandle such structures, it would never be a serious production tool. You mightuse it for a reporting tool or something off to the side, but not as the maindatabase for the enterprise. Well, IMS is still out there and it holds a lot of data, but SQL has movedinto serious production databases. As it turned out, the problem was not thatSQ 
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