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）：
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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
更多关于mysql相关内容感兴趣的读者可查看本站专题：《mysql常用函数大汇总》、《mysql日志操作技巧大全》、《mysql事务操作技巧汇总》、《mysql存储过程技巧大全》及《mysql数据库锁相关技巧汇总》
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