Querying Incomplete Information Using Bag Relational Algebra

In the paper, we introduce bag relational algebra with grouping and aggregation overa particular representation of incomplete information called c-tables, which wasfirst introduced byGrahne in 1984.In order for this algebra to be closed and ``well-defined´´, we adopt the closed world assumptionas described by Reiter in 1978 and extend the tuple and table conditions to linear ones.It turns out that query answering overthe described extension remains polynomial timerelative to the size of the certain information. Therefore, the proposed algebra can be implemented as partof a SQL engine that can query incomplete information. The execution time will be acceptable as long as the size of the incomplete information issmall relative to the total size of the database.