错误信息：System.Data.SqlClient.SqlException: 传递到 SUBSTRING 函数的长度参数无效

数据库执行下面的sql 没问题，但是用。net 获取datatable 或者用dataread去读取的时候，会报错误  具体的：LEFT(contxt,LEN(contxt)-1

如果把left去掉，不报错，取出来的 contxt是空，是我字符串太长了吗？

with student as
( SELECT TOP 100 PERCENT t1.UserID ,t2.UserName,t2.[group],t2.sort,t1.StyleImagePath+'^'+t1.StyleCode+'^'+CONVERT(VARCHAR(10),XYDdate,120) +'^'
+CONVERT(VARCHAR(10),DBWCdate,120)+'^'+MLDYdate+'^'+MLDLdate+'^'+FLDYdate+'^'+CONVERT(VARCHAR(10),FZWCdate,120)+'^'+Remark AS contxt
FROM [dy_result] t1 LEFT JOIN dy_user t2 ON t1.UserID=t2.UserID
ORDER BY t2.[Group],t2.sort asc )
SELECT UserID,UserName,CAST(LEFT(contxt,LEN(contxt)-1) AS Text) as contxt FROM(
SELECT [group],sort,UserID,UserName,
(SELECT contxt+'$' FROM student
WHERE UserID=A .UserID AND UserName=A.UserName
FOR XML PATH('')) AS contxt
FROM student A GROUP BY [group],sort,UserID,UserName) B ORDER BY [group],sort