本文例項講述了mysql多個left join連線查詢用法。分享給大家供大家參考，具體如下：

mysql查詢時需要連線多個表時，比如查詢訂單的商品表，需要查詢商品的其他資訊，其他資訊不在訂單的商品表，需要連線其他庫的表，但是連線的條件基本都是商品ID就可以了，先給一個錯誤語句（查詢之間的巢狀，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,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,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

查詢結果耗時4秒多，explain分析，發現其中2個子查詢是全部掃描，可以使用mysql的多個left join優化

SELECT
  A.sub_order_id,(
    A.sell_price - A.supply_price
  ) AS sell_profit,(
    A.sell_price - A.supply_price
  ) / A.sell_price AS sell_percent,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

查詢結果耗時0.04秒

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