  Query response time often influences user experience in the real world.
However, it possibly takes more time to answer a query with its all exact
solutions, especially when it contains the OPT operations since the OPT
operation is the least conventional operator in SPARQL. So it becomes essential
to make a trade-off between the query response time and the accuracy of their
solutions. In this paper, based on the depth of the OPT operation occurring in
a query, we propose an approach to obtain its all approximate queries with less
depth of the OPT operation. This paper mainly discusses those queries with
well-designed patterns since the OPT operation in a well-designed pattern is
really "optional". Firstly, we transform a well-designed pattern in OPT normal
form into a well-designed tree, whose inner nodes are labeled by OPT operation
and leaf nodes are labeled by patterns containing other operations such as the
AND operation and the FILTER operation. Secondly, based on this well-designed
tree, we remove "optional" well-designed subtrees with less depth of the OPT
operation and then obtain approximate queries with different depths of the OPT
operation. Finally, we evaluate the approximate query efficiency with the
degree of approximation.
