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CQL is a query language used by SRU.

About CQL

as described in the CQL specification:

CQL, the Contextual Query Language, is a formal language for representing queries to information retrieval systems such as web indexes, bibliographic catalogs and museum collection information. The design objective is that queries be human readable and writable, and that the language be intuitive while maintaining the expressiveness of more complex languages.
Traditionally, query languages have fallen into two camps: Powerful, expressive languages, not easily readable nor writable by non-experts (e.g. SQL, PQF, and XQuery);or simple and intuitive languages not powerful enough to express complex concepts (e.g. CCL and google). CQL tries to combine simplicity and intuitiveness of expression for simple, every day queries, with the richness of more expressive languages to accomodate complex concepts when necessary.