What's the easiest way to select a single record/value from the n-th group? The group is determined by a material and it's price(prices can change). I need to find the first date of the last and the last date of the next to last material-price-groups. So i want to know when exactly a price changed. What's the easiest way to select a single recor
阅读背景：
获取第n个连续组的第一行/最后一行
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