Several databases give free access now
نویسندگان
چکیده
منابع مشابه
Querying several conflictual databases
This paper addresses the problem of querying several databases considered as a whole. Assuming that the different databases share a common data description language, the problem that raises is to consistently answer queries even if the database contents are contradictory. The main contribution of this paper is the specification of a query-evaluator for answering closed and open queries addresse...
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Most real-world databases record time-varying information. In such databases, the notion of “the current time,” or NOW , occurs naturally and prominently. For example, when capturing the past states of a relation using begin and end time columns, tuples that are part of the current state have some past time as their begin time and NOW as their end time. While the semantics of such variable data...
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ژورنال
عنوان ژورنال: BMJ
سال: 2002
ISSN: 0959-8138,1468-5833
DOI: 10.1136/bmj.324.7340.790/a