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In temporal databases, transaction time (TT) is the time period during which a fact stored in the database is considered to be true. As of December 2011, ISO/IEC 9075, Database Language SQL:2011 Part 2: SQL/Foundation included clauses in table definitions to define "system-versioned tables" (that is, transaction-time tables).
When the ending transaction time is unknown, it may be considered as "Until Changed". Academic researchers and some RDBMS have represented "Until Changed" with the largest timestamp supported or the keyword "forever". This convention is not technically precise.
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