mysql多个left join连接查询用法分析
本文实例讲述了mysql多个leftjoin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_idASorder_id, A.photo_idASwid, A.photo_nameASwork_name, A.supply_price, A.sell_price, sum(A.num)AStotal_num, ( A.sell_price-A.supply_price )ASsell_profit, ( A.sell_price-A.supply_price )/A.sell_priceASsell_percent, A.goods_id, A.goods_name, B.goods_nameASclassify FROM order_goodsASA LEFTJOIN( SELECT A.goods_id, A.parentid, B.goods_name FROM test_qyg_goods.goodsASA LEFTJOINtest_qyg_goods.goodsASBONA.parentid=B.goods_id )ASBONA.goods_id=B.goods_id WHERE A.createtime>='2016-09-0900:00:00' ANDA.createtime<='2016-10-1623:59:59' ANDFROM_UNIXTIME( UNIX_TIMESTAMP(A.createtime), '%Y-%m-%d' )!='2016-09-28' ANDFROM_UNIXTIME( UNIX_TIMESTAMP(A.createtime), '%Y-%m-%d' )!='2016-10-07' GROUPBY A.photo_id ORDERBY A.goods_idASC )ASA LEFTJOIN( SELECT A.wid, A.brand_id, B.gb_name FROM test_qyg_user.buser_goods_listASA LEFTJOINtest_qyg_supplier.brandsASBONA.brand_id=B.gbid )ASBONA.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leftjoin优化
SELECT A.sub_order_id, A.photo_idASwid, A.photo_nameASwork_name, A.supply_price, A.sell_price, sum(A.num)AStotal_num, ( A.sell_price-A.supply_price )ASsell_profit, ( A.sell_price-A.supply_price )/A.sell_priceASsell_percent, A.goods_id, A.goods_name, B.parentid, C.goods_nameASclassify, D.brand_id, E.gb_name, sum( CASE WHENF.buy_type='yes'THEN A.num ELSE 0 END )AStotal_buy_num, sum( CASE WHENF.buy_type='yes'THEN A.num ELSE 0 END*A.sell_price )AStotal_buy_money, sum( CASE WHENF.buy_type='no'THEN A.num ELSE 0 END )AStotal_give_num, sum( CASE WHENF.buy_type='no'THEN A.num ELSE 0 END*A.sell_price )AStotal_give_money FROM order_goodsASA LEFTJOINtest_qyg_goods.goodsASBONA.goods_id=B.goods_id LEFTJOINtest_qyg_goods.goodsASCONB.parentid=C.goods_id LEFTJOINtest_qyg_user.buser_goods_listASDONA.photo_id=D.wid LEFTJOINtest_qyg_supplier.brandsASEOND.brand_id=E.gbid LEFTJOINorder_info_subASFONA.sub_order_id=F.order_id WHERE A.createtime>='2016-09-0900:00:00' ANDA.createtime<='2016-10-1623:59:59' ANDFROM_UNIXTIME( UNIX_TIMESTAMP(A.createtime), '%Y-%m-%d' )!='2016-09-28' ANDFROM_UNIXTIME( UNIX_TIMESTAMP(A.createtime), '%Y-%m-%d' )!='2016-10-07' GROUPBY A.photo_id ORDERBY A.goods_idASC
查询结果耗时0.04秒
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