How to combine "SELECT" and "REPLACE INTO" when combining two MySQL tables into another?
as variant:

INSERT INTO t3 


SELECT t1.measurement_id,
CASE WHEN t1.value IS NULL THEN
     t2.value
ELSE 
     t1.value
END as value

FROM t1 LEFT JOIN t2 ON t2.measurement_id = t1.measurement_id AND t1.value IS NULL

UNION ALL

SELECT 
     t2.measurement_id,
     t2.value 
FROM t2 WHERE t2.measurement_id NOT IN (SELECT measurement_id FROM t1)


as benefits - no REPLACE