async fetchAllDeltas() {
        const knex = this.knex;
        const ninetyDaysAgo = moment.utc().subtract(90, 'days').startOf('day').utc().format('YYYY-MM-DD HH:mm:ss');
        const rows = await knex('members_paid_subscription_events')
            .select('currency')
            // In SQLite, DATE(created_at) would map to a string value, while DATE(created_at) would map to a JSDate object in MySQL
            // That is why we need the cast here (to have some consistency)
            .select(knex.raw('CAST(DATE(created_at) as CHAR) as date'))
            .select(knex.raw(`SUM(mrr_delta) as delta`))
            .where('created_at', '>=', ninetyDaysAgo)
            .groupByRaw('CAST(DATE(created_at) as CHAR), currency');
        return rows;
    }
Get the MRR deltas for all days (from old to new), grouped by currency (ascending alphabetically)
@returns {Promise<MrrDelta[]>} The deltas sorted from new to old