sharepoint list item without entry in required fields

i have a sharepoint 开发者_JAVA技巧list which has few rows without having any entry in required fields .

are there any ways to populate a item without required fields?..i need to know the reason how those items were made up without required fields.

could some one please tell me how this could have happened.

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In addition, I don't think all of those checks are enforced when manipulating items via the object model or web services.

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• If the column was optional when the item has been uploaded/created.
• If you uploaded a xls spreadshead
• If you uploaded multiple documents at a single blow