  The rise of open and permissionless blockchains has introduced novel
platforms for applications based on distributed data storage. At the
application and business levels, long-established query languages such as SQL
provide interoperability that can be complemented by blockchain-based data
storage today, enabling permissionless and verifiable data storage along with
decentralized execution across tens of thousands of nodes. However, when
accessing one or more blockchains, interoperability is not provided today,
posing challenges such as inhomogeneous data access in addition to different
features and trade-offs, e.g. in data and distribution, scalability, and
security. Towards interoperability in data access among the increasing number
of blockchain platforms, this paper introduces a cross-chain query language for
data access across blockchains. Similar to SQL, the language abstracts from
implementation based on a data model compatible with the largest open and
permissionless blockchains (OPB) today. The language, data model, and
processing architecture are demonstrated and evaluated with an implemented
prototype, aiming to contribute to the discussion on blockchain
interoperability among OPB.
