本文實例講述了mysql多個left join連接查詢用法。分享給大家供大家參考，具體如下：
mysql查詢時需要連接多個表時，比如查詢訂單的商品表，需要查詢商品的其他信息，其他信息不在訂單的商品表，需要連接其他庫的表，但是連接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_nameFROM ( 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 ALEFT 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.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_moneyFROM order_goods AS ALEFT JOIN test_qyg_goods.goods AS B ON A.goods_id = B.goods_idLEFT JOIN test_qyg_goods.goods AS C ON B.parentid = C.goods_idLEFT JOIN test_qyg_user.buser_goods_list AS D ON A.photo_id = D.widLEFT JOIN test_qyg_supplier.brands AS E ON D.brand_id = E.gbidLEFT JOIN order_info_sub AS F ON A.sub_order_id = F.order_idWHERE 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_idORDER BY A.goods_id ASC
查詢結果耗時0.04秒
希望本文所述對大家MySQL數據庫計有所幫助。
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