Collection of usage of DB class in codeigniter

Link database
-------
$this->load->database();//Manually connect to the database
//Connect multiple databases
$DB1 = $ this->load->database('group_one', TRUE);
$DB2 = $this->load->database('group_two', TRUE);
Query
-------
//Parameter binding form
$sql = "SELECT * FROM some_table WHERE id = ? AND status = ? AND author = ?";
$this->db->query( $sql, array(3, 'live', 'Rick'));
//Multi-result standard query
$query = $this->db->query($sql); //Customized
$query = $this->db->get('tablename'); //Convenient form, equivalent to: SELECT * FROM tablename
$query = $this->db->get('tablename', 10, 20); // Equivalent to: SELECT * FROM tablename LIMIT 20, 10
$query->result() //Object form
$query->result_array() //Array form

$query->num_rows () //Total number of items
$query->num_fields() //Number of fields
//Single result standard query
$row = $query->row(); //Object form
$row = $query ->row_array(); //Array form

insert
-------
$data = array(
 'title' => $title,
 'name' => $name
 );
$this->db->insert('tablename', $data); //Convenient insertion
$this->db->insert_string('tablename', $data); //Convenient insertion
$ this->db->insert_id() //The id just inserted
$this->db->affected_rows() //The number of affected rows (update,insert)
-------
$data = array(
 'name' => $name,
 'email' => $email
 );
$where = "id = 1";
$ this->db->update('tablename', $data);
$this->db->update_string('tablename', $data, $where);
-------
$array = array(
 'name' => $name,
 'title' => $title
 );
$this->db->delete( 'tablename', $array);
// Produces:
// "DELETE FROM tablename WHERE name = '$name' AND title = '$title'"
$this->db->truncate('tablename' ); //Clear the table
// Produce: TRUNCATE tablename
（where）
-------
$array = array(
 'name' => $name,
 'title' => $title
 );
$this->db->where($array);
// Produces: "WHERE name = '$name' AND title = '$title'"
 -------------------------------------------------- ---
$this->db->count_all('tablename'); //The total number of rows recorded in the table
---------------------- -------------------------------
$query->free_result() //Release resources