I read a description from When to Use MEMORY or NDB Cluster from the MySQL website.
Operations involving transient, non-critical data such as session management or caching. When the MySQL server halts or restarts, the data in MEMORY tables is lost.
but my created tables with ENGINES = MEMORY were not dropped upon mysql server restart.
COLUMN_1 varchar(255) DEFAULT NULL,
COLUMN_2 varchar(255) DEFAULT NULL,
COLUMN_3 varchar(255) DEFAULT NULL
) ENGINE=MEMORY DEFAULT CHARSET=utf8;