does this work for you ...?

with cte as

(

SELECT iphone_id, MAX(date_assigned) AS Maxd

FROM location_history

GROUP BY iphone_id

)

SELECT iPhones.iphone_id,

iPhones.seriel_number,

cte.Maxd,

locations.Emp_name,

locations.Department

FROM iPhones

INNER JOIN cte ON iPhones.iphone_id = cte.iphone_id

INNER JOIN location_history ON cte.iphone_id = location_history.iphone_id

AND cte.Maxd = location_history.date_assigned

INNER JOIN locations ON location_history.location_id = locations.location_id