What are the different structural approaches to implementing CQRS, from simplest to most complete?

There are five real structural flavors. The simplest is a single shared model on a single database, which only separates the API into write and read paths without gaining CQRS's other advantages. Next is separate command and query models sharing one database, which still commits changes atomically but lets each side use a model shaped for its own job. Third is separate models on separate databases without event sourcing, which allows independent scaling but risks events arriving at the read side out of order. Fourth is CQRS combined with event sourcing on separate databases, which fixes that ordering risk through strict per-aggregate sequencing and enables reliable replay and independent scaling, at the cost of real operational complexity. Fifth is event sourcing with the event store and read model sharing one database, trading independent scaling for stronger, immediate consistency while keeping most of event sourcing's benefits.