How does Elixir's Ecto handle complex queries and associations?

Ecto's query DSL handles complex SQL patterns. Composable queries: build queries incrementally: query = from u in User; query = if filter_active, do: where(query, [u], u.active == true), else: query; Repo.all(query). Associations: define in schema: has_many :posts, Post; belongs_to :user, User. Load: Repo.preload(user, :posts) or with query: from u in User, preload: [:posts]. Join: from u in User, join: p in Post, on: p.user_id == u.id, select: {u, p}. Aggregates: from o in Order, group_by: o.status, select: {o.status, count(o.id)}. Fragments: raw SQL when needed: fragment("date_trunc('month', ?)", p.inserted_at). Schemaless queries: query without schema for reporting: Repo.all(from "orders", select: [:id, :total]). Multi-tenancy: prefix schemas with Repo.all(query, prefix: tenant_id). Ecto's separation of schema, changeset, and query makes complex database access patterns manageable while maintaining type safety.