Two Ways To Count

Table readings:

id	sensor	value
1	a	10
2	a	NULL
3	a	20
4	b	NULL

SELECT sensor,
       count(*)     AS c_star,
       count(value) AS c_val,
       sum(value)   AS total,
       avg(value)   AS mean
FROM readings
GROUP BY sensor
ORDER BY sensor;

What are the two output rows?