题解 | 电商平台需要对商家的销售业绩、退款情况和客户满意度进行综合评估

关键：进行子查询后连接，避免原表连接导致笛卡尔积膨胀。

select m.merchant_id merchant_id,merchant_name,total_sales_amount,total_refund_amount,average_satisfaction_score
from merchants_underline m
left join (
    select merchant_id,sum(sale_amount) total_sales_amount
    from sales_underline
    group by merchant_id
) s on m.merchant_id=s.merchant_id
left join (
    select merchant_id,sum(refund_amount) total_refund_amount
    from refunds_underline
    group by merchant_id
) r on m.merchant_id=r.merchant_id
left join (
    select merchant_id,round(avg(satisfaction_score),2) average_satisfaction_score
    from satisfaction_underline
    group by merchant_id
) sa on m.merchant_id=sa.merchant_id
order by merchant_id