The Generic REST Adapter supports standard SQL joins like the following examples.

Inner Join

An inner join selects only rows from both tables that match the join condition:

SELECT 
  "people"."personal.age" AS age, 
  "people"."personal.gender" AS gender, 
  "people"."personal.name.first" AS first_name, 
  "people"."personal.name.last" AS last_name, 
  "vehicles"."model", 
FROM 
  "people", "vehicles" 
WHERE 
  "people"."_id" = "vehicles"."people_id"

Left Join

A left join selects all rows in the FROM table and only matching rows in the JOIN table:

SELECT 
  "people"."personal.age" AS age, 
  "people"."personal.gender" AS gender, 
  "people"."personal.name.first" AS first_name, 
  "people"."personal.name.last" AS last_name, 
  "vehicles"."model", 
FROM 
  "people"
LEFT OUTER JOIN 
  "vehicles" 
ON
  "people"."_id" = "vehicles"."people_id"