分组不是基于起始元素，相反，有时需要对彼此相邻的项进行分组。
Instead of grouping based on a beginning element, it is sometimes necessary to group items that are adjacent to each other.
虽然索引项是连续的，但是不保证包含相邻索引项的行将驻留在同一数据页上。
Although index entries are sequential, there is no guarantee that rows with adjacent index entries will reside on the same data pages.
uniq(读做“unique”)，扫描列表，比较列表中的相邻元素，以去除任何重复项。
Uniq (pronounced "unique") scans a list, comparing adjacent elements in the list and removing any duplicates. For instance, suppose you have this list.
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