The CData Excel Add-In for REST 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]