FluidDB provides a simple query language that allows applications to search for objects based on their tags’ values. The following kinds of queries are possible:

Numeric: To find objects based on the numeric value of tags. For example, tim/rating > 5.
Textual: To find objects based on text matching of their tag values, e.g., sally/opinion matches fantastic. Text matching is done with Lucene, meaning that Lucene matching capabilities and style will be available (coming soon).
Presence: Use has to request objects that have a given tag. For example, has sally/opinion.
Set contents: A tag on an object can hold a set of strings. For example, a tag called mary/product-reviews/keywords might be on an object with a value of [ "cool", "kids", "adventure" ]. The contains operator can be used to select objects with a matching value. The query mary/product-reviews/keywords contains "kids" would match the object in this example.
Exclusion: You can exclude objects with the except keyword. For example has nytimes.com/appeared except has james/seen. The except operator performs a set difference.
Logic: Query components can be combined with and and or. For example, has sara/rating and tim/rating > 5.
Grouping: Parentheses can be used to group query components. For example, has sara/rating and (tim/rating > 5 or mike/rating > 7).

That's it!

More information can be found here: http://doc.fluidinfo.com/fluidDB/queries.html