This is similar to the previous query in terms of the information we require in order to obtain the average:

    const getAverageOfReadingsBetweenTime = (type, start, end,
         callback) => {
      db.get(`SELECT avg(value) FROM temperature WHERE     
    createdAt > ? AND createdAt < ?;`, [start, end], callback)
    }

In this function, we're using the get method instead of the all method since we expect only one reading to be returned.