Codeigniter won’t prepend “N” to strings and that’s critical for SQL server to retrieve string values. The solution is to revise DB driver of CI:

File: sys\database\DB_Driver.php

	function escape($str)
	{
		if (is_string($str))
		{
			//Revise this part
			if($this->dbdriver == 'sqlsrv'){
				$str = "N'".$this->escape_str($str)."'";
			}else{
				$str = "'".$this->escape_str($str)."'";
			}
		}
		elseif (is_bool($str))
		{
			$str = ($str === FALSE) ? 0 : 1;
		}
		elseif (is_null($str))
		{
			$str = 'NULL';
		}

		return $str;
	}