Hour() ser ud til at det ville være en nyttig funktion til dette, da du kun kigger på en enkelt dag. Måske noget som dette ville fungere for dig:
SELECT * FROM
(SELECT HOUR(time) hour, avg(ph) AS avg_ph
FROM ph
WHERE time >= NOW() - INTERVAL 1 DAY
GROUP BY hour) p
JOIN
(SELECT HOUR(time) hour, avg(temperature) AS avg_temp
FROM temperature1
WHERE time >= NOW() - INTERVAL 1 DAY
GROUP BY hour) t ON t.hour = p.hour
JOIN
(SELECT HOUR(time) hour, avg(solids) AS avg_solids
FROM solids
WHERE time >= NOW() - INTERVAL 1 DAY
GROUP BY hour) s ON s.hour = p.hour;
Da det bruger indre joinforbindelser, antager jeg, at der altid vil være mindst én post i hver tabel for timen, men det virker som en rimelig antagelse.