How to reference class files in idea

idea"s Java project adds a single class file to the lib directory, but does not know how to add a reference to the path in the project?

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Class java idea
Mar.21,2021

"File"-> "Project Structure"-> "Library", click "+"

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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-64f5-1b30a19-2bd27.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b30a19-2bd27.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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