可以left join 之后加and  条件。这样查询出来的记录不会少，为空的也能查出来，但是仍然能筛选

 

 

 

例子：

select t.product_id,
       t.unit_price as price,
       t.number,
       t.item_total_price,
       t.remark,
       t.im_cost_price,
       t.im_cost_price_intax,
       t.create_user_id,
       t.item_currency,
       p.sku,
       p.belong_stock,
       p.cost_price,
       o.order_no,
       s.product_address as photoUrl,
       s.show_kind,
       s.is_main_photo
from trade_order_item t left join trade_product_v2 p on t.product_id = p.product_id left join trade_product_show s on t.product_id = s.product_id
and s.is_main_photo = 0

这里trade_product_show

这里表的is_main_photo 字段不是所有记录都有值，所以不能在where子句上筛选 所以 left join 之后加and

select t.product_id,
       t.unit_price as price,
       t.number,
       t.item_total_price,
       t.remark,
       t.im_cost_price,
       t.im_cost_price_intax,
       t.create_user_id,
       t.item_currency,
       p.sku,
       p.belong_stock,
       p.cost_price,
       o.order_no,
       s.product_address as photoUrl,
       s.show_kind,
       s.is_main_photo
from trade_order_item t left join trade_product_v2 p on t.product_id = p.product_id left join trade_product_show s on t.product_id = s.product_id
and s.is_main_photo = 0