int 0什么时候比unsigned int－1小？（C语言）

#include <stdio.h>
#include <stdlib.h>

int main()
{
        unsigned int a = 0;
        int b =a-1;
        if(a>b)
            printf("yes!");
        else

            printf("no!%d\n",sizeof(a));

        return 0;
}