给商品id和用户id建立唯一索引
在sql中加入count>0条件,由于update是一种排他锁,因此只会有一个线程进入,这时只要判断库存大于0就行了 update goods set num = num - 1 WHERE id = 1001 and count > 0
或是CAS的原理建立一个version列 修改前先查询version,update修改版本号和库存where版本号是对的同时库存大于0。
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