左连接查询（LEFT JOIN）注意事项

• 左连接查询条件（ON）要确保条件唯一，否则会生成多个值

例：

select c.card_id,cc.card_number from `vip_card_change` as cc left join `vip_card` as c on cc.card_number=c.card_number and cc.store_key=c.store_key;

而不是：

select c.card_id,cc.card_number from `vip_card_change` as cc left join `vip_card` as c on cc.card_number=c.card_number;