Views Aggregation Problems
For a short tutorial about the "aggregation" features of the Views module, refer to Using aggregation in Views. As an alternative, have a look at the (free) set of 30 video tutorials about the Views module. Specific to "Aggregation", you may want to look at these episodes (quotes are from the linked pages):


The aggregation setting.


  ... makes Views return not one row per item in the database, but one row per aggregation group. Perhaps more importantly, it also makes it possible to count the number of items baked into each row, or sum them up, or calculate their average, or count the number of unique items, or…

Aggregation and relationships.


  ... shows how to use the aggregation setting in Views in combination with relationships.

Aggregation edge cases.


  ... shows some of the more rare use cases of aggregations in Views:
  
  
  Using aggregation on filtering
  Using aggregation on sorting
  Using aggregation on contextual filters (which is currently broken)
  NOT aggregation on relationships (since aggregation doesn't apply to this handler)
  Using aggregation on multiple-value fields