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When defining a Drupal content type, I continually come into the situation where I want to add fields to my content type as a group. That is, creating a CCK field called “Contact Info", which is a group containing Contact Name and Contact Email, for instance. With regular CCK this has to be done by adding several separate fields or by creating another content type for that “Contact Info” group. However, a while ago I learned how to create a compound CCK field that is essentially a group container of fields you define.
Drupal has an inconsistent data structure for CCK fields: if a field is only in one content type, it's stored in a content_type_XX table as a column, but if it's shared across multiple content types, it moves to its own content_field_XX column. Views figures out where fields are located automatically, but in custom SQL queries, this can be a real pain - you can write a query that works one day, then share a field, and the query breaks.