The example uses JSON_KEYS to get an ordered list of all unique keys from all documents of all rows of a table.
SELECT distinct(JSONKeys) from JSON_KEYS ( ON (SELECT json_data FROM json_table)) AS json_data ORDER BY 1;
Result:
JSONKeys -------- "base" "clouds" "clouds"."all" "cod" "coord" "coord"."lat" "coord"."lon" "dt" "id" "main" "main"."humidity" "main"."pressure" "main"."temp" "main"."temp_max" "main"."temp_min" "main"."temp_scale" "name" "sys" "sys"."country" "sys"."sunrise" "sys"."sunset" "weather" "weather"[0] "weather"[0]."description" "weather"[0]."icon" "weather"[0]."id" "weather"[0]."main" "wind" "wind"."deg" "wind"."speed"