The best quick overview I've seen of SimpleDB is from Working with Amazon SimpleDB - CodeProject:

When getting data from SimpleDB, we need to remember four things:

  • Amazon SimpleDB does not have relationships, so a query can only access one domain, and can not join.
  • All attribute value returns have the type string.
  • In SimpleDB, there is no requirement for each row to have the same set of attributes. So when you get an item that does not have value for an attribute, you might think that in the result item, that attribute will have a blank value or null, but that is wrong. The result item will not have that attribute in the set of attributes and we cannot get that value.
  • When you use "get data by query", the column ID cannot be used to filter, sort... because the ID column is a unique column that refers to the name of the item and it does not exist in the attribute of the item.

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