Is relational algebra a procedural, imperative, and/or declarative language?
The terminology used in the database area calls the relational algebra “procedural” to contrast it with the languages based on “calculus”, since an algebraic expression describes an ordered set of steps to find the result: simply execute the operations in the correct order to produce the result. 

In contrast, in an expression of a calculus based language, the result is described through a property that holds on it, without specifying which operations should be applied to find it, neither the order in which they should be applied. So it “declares” the properties of the result, not a “procedure” to obtain it.

This terminology has no particular relations with other terminologies in the field of programming languages (like imperative, functional, applicative, etc.)