1、自连接

    select * from product where vender_id = (select verder_id from product where name='Dove')

    select p1.* from product p1, product p2 where p1.id = p2.id and p2.name = 'Dove'

2、根据同表字段更新

    update product p1,product p2  set p1.photo_status = 2

    where p1.id = p2.id and p2.photo_status is null

3、连接要有条件，否则将出现笛卡尔积