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
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b363a2-2bff2.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b363a2-2bff2.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?