10.4. Collection Types

Data modeling approaches vary considerably in their support for collection types. While implementation targets often support such types (e.g., SQL:2003 supports arrays, row types, and bags), high level data modeling approaches typically provide either no direct support or only very limited direct support. Even when no direct support for fact collections is provided, it is possible to model the same information in terms of elementary facts, so the issue of collection type support is one of convenience rather than expressibility. The following collection types are the most commonly encountered in conceptual modeling:

  • Set (unordered, no duplicates)

  • Bag or multiset (unordered, duplicates allowed)

  • Ordered set or unique sequence ...

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