mysql多个left join连接查询用法分析

本文实例讲述了mysql多个left join连接查询用法。分享给大家供大家参考，具体如下：

mysql查询时需要连接多个表时，比如查询订单的商品表，需要查询商品的其他信息，其他信息不在订单的商品表，需要连接其他库的表，但是连接的条件基本都是商品id就可以了，先给一个错误语句（查询之间的嵌套，效率很低）：

select
  a.order_id,
  a.wid,
  a.work_name,
  a.supply_price,
  a.sell_price,
  a.total_num,
  a.sell_profit,
  a.sell_percent,
  a.goods_id,
  a.goods_name,
  a.classify,
  b.gb_name
from
  (
    select
      a.sub_order_id as order_id,
      a.photo_id as wid,
      a.photo_name as work_name,
      a.supply_price,
      a.sell_price,
      sum(a.num) as total_num,
      (
        a.sell_price - a.supply_price
      ) as sell_profit,
      (
        a.sell_price - a.supply_price
      ) / a.sell_price as sell_percent,
      a.goods_id,
      a.goods_name,
      b.goods_name as classify
    from
      order_goods as a
    left join (
      select
        a.goods_id,
        a.parentid,
        b.goods_name
      from
        test_qyg_goods.goods as a
      left join test_qyg_goods.goods as b on a.parentid = b.goods_id
    ) as b on a.goods_id = b.goods_id
    where
      a.createtime >= '2016-09-09 00:00:00'
    and a.createtime <= '2016-10-16 23:59:59'
    and from_unixtime(
      unix_timestamp(a.createtime),
      '%y-%m-%d'
    ) != '2016-09-28'
    and from_unixtime(
      unix_timestamp(a.createtime),
      '%y-%m-%d'
    ) != '2016-10-07'
    group by
      a.photo_id
    order by
      a.goods_id asc
  ) as a
left join (
  select
    a.wid,
    a.brand_id,
    b.gb_name
  from
    test_qyg_user.buser_goods_list as a
  left join test_qyg_supplier.brands as b on a.brand_id = b.gbid
) as b on a.wid = b.wid

查询结果耗时4秒多，explain分析，发现其中2个子查询是全部扫描，可以使用mysql的多个left join优化

select
  a.sub_order_id,
  a.photo_id as wid,
  a.photo_name as work_name,
  a.supply_price,
  a.sell_price,
  sum(a.num) as total_num,
  (
    a.sell_price - a.supply_price
  ) as sell_profit,
  (
    a.sell_price - a.supply_price
  ) / a.sell_price as sell_percent,
  a.goods_id,
  a.goods_name,
  b.parentid,
  c.goods_name as classify,
  d.brand_id,
  e.gb_name,
  sum(
    case
    when f.buy_type = 'yes' then
      a.num
    else
      0
    end
  ) as total_buy_num,
  sum(
    case
    when f.buy_type = 'yes' then
      a.num
    else
      0
    end * a.sell_price
  ) as total_buy_money,
  sum(
    case
    when f.buy_type = 'no' then
      a.num
    else
      0
    end
  ) as total_give_num,
  sum(
    case
    when f.buy_type = 'no' then
      a.num
    else
      0
    end * a.sell_price
  ) as total_give_money
from
  order_goods as a
left join test_qyg_goods.goods as b on a.goods_id = b.goods_id
left join test_qyg_goods.goods as c on b.parentid = c.goods_id
left join test_qyg_user.buser_goods_list as d on a.photo_id = d.wid
left join test_qyg_supplier.brands as e on d.brand_id = e.gbid
left join order_info_sub as f on a.sub_order_id = f.order_id
where
  a.createtime >= '2016-09-09 00:00:00'
and a.createtime <= '2016-10-16 23:59:59'
and from_unixtime(
  unix_timestamp(a.createtime),
  '%y-%m-%d'
) != '2016-09-28'
and from_unixtime(
  unix_timestamp(a.createtime),
  '%y-%m-%d'
) != '2016-10-07'
group by
  a.photo_id
order by
  a.goods_id asc

查询结果耗时0.04秒

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