@article{DBLP:journals/jcss/ChandraH82,
author = {Ashok K. Chandra and
David Harel},
title = {Structure and Complexity of Relational Queries},
journal = {J. Comput. Syst. Sci.},
volume = {25},
number = {1},
year = {1982},
pages = {99-128},
ee = {http://dx.doi.org/10.1016/0022-0000(82)90012-5},
bibsource = {DBLP, http://dblp.uni-trier.de}
}