https://support.microsoft.com/zh-cn/help/189897/data-truncated-to-255-characters-with-excel-odbc-driver
Microsoft.ACE.OLEDB.12.0 驱动读取excel，会读取前8行来判定每列的数据类型，假如没有超过255个字符，那么会被设置为nvarchar(255),从第9行开始，超过255字符会被自动截断！
推荐使用第三方库：https://github.com/ExcelDataReader/ExcelDataReader
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附连接字符串
Microsoft ACE OLEDB 12.0
Provider=Microsoft.ACE.OLEDB.12.0;Data Source=c:\myFolder\myExcel2007file.xlsx;
Extended Properties="Excel 12.0 Xml;HDR=YES";
驱动下载Microsoft Access Database Engine 2016 Redistributable
https://www.microsoft.com/en-us/download/details.aspx?id=54920