varchar相关

https://galaxyyao.github.io/2019/07/30/MySQL-%E6%B2%A1%E6%9C%89%E5%BF%85%E8%A6%81%E7%9A%84varchar-255-%E9%95%BF%E5%BA%A6%E5%8F%8A%E5%AD%98%E5%82%A8%E6%B1%89%E5%AD%97%E9%97%AE%E9%A2%98%E6%B1%87%E6%80%BB/