In css, the height of the parent element is not fixed, how to achieve the height of the child element 100%

it"s a bit of an idiot question. It is really a brain short circuit can not remember, please give us a method, the parent element is through the number of data li has been pushed down, so can not be fixed height, so there is a div on the left how to make its height is always 100% of the parent element?

Css
Mar.05,2021
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