Linq高级查

开头：StartsWith()
结尾：EndsWith()
模糊：Contains()

个数：Count
最大值：Max(r => r.price)
最小值：Min(r => r.price)
平均值：Average(r => r.price)
求和：Sum(r => r.price)

升序：OrderBy(r => r.price);
降序：OrderByDescending(r => r.price)

分页：Skip(PageCount * (a - 1)).Take(PageCount)