[ghislainfourny]

Thank you for your question.

This is, in fact, not an assignment, but a variable binding that is highly optimizable by an execution engine. The let clause is part of the FLWOR expression, works in orchestration with for, where, group by, order by, count, return; the ability to bind variables while doing relational algebra is a feature that is often missed in SQL.

JSONiq is functional and, in its core (non scripting) version, does not allow modifying variable values.