语法上没有问题，看输出语句后，检查下关键词间的空格吧

把拼接好的sql语句打印看一下，我怀疑是inner 和where的空格问题

两个字符串连接了，但是中间没有空格呀Employeesinner这样肯定设别不错来，不要这样写，容易错

string sql = "select EmployeeId,EmployeeName,Gender,Birthday,DepartmentName from Employees";
sql += "inner join EmployeeDepartment on Employees.DepartmentId=EmployeeDepartment.DepartmentId";
sql += "where Employees.DepartmentId="+departmentId;

string sql复用了吧？
string sql = "select EmployeeId,EmployeeName,Gender,Birthday,DepartmentName from Employees"

• "inner join EmployeeDepartment on Employees.DepartmentId=EmployeeDepartment.DepartmentId"+ "where Employees.DepartmentId="+departmentId; 不知道对不对？

执行完后的sql语句是这样的:
select EmployeeId,EmployeeName,Gender,Birthday,DepartmentName from Employeesinner join EmployeeDepartment on Employees.DepartmentId=EmployeeDepartment.DepartmentIdwhere Employees.DepartmentId=1
问题在拼接的地方没有空格隔开.

StringBuilder sql = new StringBuilder(" select EmployeeId,EmployeeName,Gender,Birthday,DepartmentName from Employees inner join EmployeeDepartment on Employees.DepartmentId=EmployeeDepartment.DepartmentId");
sql.Append(string.Format(" where Employees.DepartmentId ={0}", departmentId));
ExcuteSql(sql.toString());