@Wikidata is no relational #database

When you consider the functionality of Wikidata, it is important to appreciate it is not a relational database. As a consequence there is no implicit way to enforce restrictions. Emulating relational restrictions fail because it is not possible to check in real time what it is that is to be restricted.

An example: in a process new items are created when there is no item available with an external identifier. Query indicates that there is no item in existence and a new item is created. A few moments later the existence of an item with the same external identifier is checked using query. Because of the time lag that exists, what is known to be in the database and what actually is in the database differs and query indicates there is no item and a new but duplicate item is created.

Implications are important.

Wikidata is a wiki. The implications are quite different. In a wiki things need not be perfect, and the restrictions of a relational model are in essence recommendations only. In such a model duplicate items as described above are not a real problem, batch jobs may merge these items when they occur often enough. Processes may use arrays knowing the items it created earlier and thereby minimising the issue.

Important is that we do not blame people for what Wikidata is not and accept its limitations. Functionality like SourceMD enable what Wikidata may become; a link to all knowledge. Never mind if it is knowledge in Wikipedia articles, scholarly articles or in sources used to prove whatever point.
Thanks,
      GerardM