The process is called Business Intelligence , the data will be stored in the data warehouse, and the business intelligence process can be used using a product such as SSAS . This process will include grouping data volumes (Who bought what and when) into data cubes . The analysis is performed on these cubes and is used to compare your purchases with others who bought the same product, then he will recommend his purchases (the other customers who bought it also bought this product .... Item X). Other various AI algorithms are used to compare models according to other customer trends, for example, how they shop, where they click, etc. All this data is accumulated and then added to the data cube for analysis.
The data mining algorithms are described below; you can find the Decision Tree Modelling algorithm , which defines BI trends and patterns (in this case, Recommendations):
http://msdn.microsoft.com/en-us/library/ms175595.aspx
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