Ozsoyoglu, Gultekin Wang, Huaiqing <br/><br/>
A Relational Calculus with  Set Operators, its Safcty, and Equivalent Graphical Languages 

 - 1038-1052 p. 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Databases<br/>Query Processing<br/>Relational Equations