SELECT b$.$id$,$ COUNT$($p$.$id$)$ AS passengers$\_$on$\_$board

FROM buses b

LEFT JOIN passengers p

ON b$.$origin $=$ p$.$origin AND b$.$destination $=$ p$.$destination

AND p$.$time $<=$ b$.$time

AND NOT EXISTS $($

SELECT $1$

FROM buses b$2$

WHERE b$2.$origin $=$ p$.$origin AND b$2.$destination $=$ p$.$destination

AND b$2.$time $<$ b$.$time AND p$.$time $<=$ b$2.$time

$)$

GROUP BY b$.$id

ORDER BY b$.$id ASC;