[drdoom]

Thanks! Now, it looks more interesting...

[devy]

Based on @aquassaut's answer, I tweaked it a little, and this should do it.

echo '[{"name":"john", "grades":[1,2,3]}, {"name":"jane", "grades":[4,5,6]}]' | jq -r '.[] | "\(.name),\(.grades|.[])"' | paste -sd' ' -