In this paper, we discuss the problem of enabling cooperative query execution in a multi-cloud environment where the data is owned and managed by multiple enterprises. We assume that each enterprise defines a set of allow rules to facilitate access to its data, which is assumed to be stored as relational tables. We propose an efficient algorithm using join properties to decide whether a given query will be allowed. We also allow enterprises to explicitly forbid access to certain data via deny rules and propose an efficient algorithm to check for conflicts between allow and deny rules.