Joomla! DBFormFields are JFormField descendants which fetch data from the database. The inspiration come from the classic master-detail implementation which I've coded twice for a project of mine. As I don't like to repeat code I prefer to implement this JFormField descendant classes to improve the project maintenance.

dbfield.php

<?php
/**
 *  Copyright (C) 2013  Pep Lainez
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *  
 * @copyright 2013, Pep Lainez
 */
defined('JPATH_PLATFORM') or die;
 
/**
 * Provides basic infrastructure for fields which fetch data from the database
 * As abstract class it cannot be instantiate directly. 
 * @author Pep Lainez
 * @package JFormFieldDBFields
 * @since    1.0
 * @see http://docs.joomla.org/Creating_a_custom_form_field_type
 */
abstract class JFormFieldDBField extends JFormField{
	
	/**
	 * Gets the query and replaces parameters
	 * using JRequest variables
	 */
	protected function prepareQuery($query){
		$regEx = "/@[a-zA-Z0-9]*/ims";
		preg_match_all($regEx, $query, $result, PREG_PATTERN_ORDER);
		if (count($result)>1){
			$values = array();
			if (count($result[1])){
				foreach($result[1] as $var){
					$values[$var] = JRequest::getString($var,'');
				}
			}
			if (count($values)){
				foreach($values as $var => $value){
					$query = str_replace($var, $value, $query);
				}
			}
		}
		return $query;
	}	
	
	/**
	 * If element is not set creates a new one
	 * (usefull for programatically access)
	 */
	private function initElement(){
		if (!isset($this->element))
			$this->element = new SimpleXMLElement('<field></field>');
	}
	
	/**
	 * Adds the columns as children of the element
	 * @param Array $columns
	 */
	public function setColumns($columns){
		$this->initElement();
		foreach($columns as $column){
			$newCol = $this->element->addChild('column');
			foreach($column as $key=>$value){
				$newCol->addAttribute($key, $value);
			}
		}
	}
	
	/**
	 * Published SimpleXMLElement addAttribute method
	 * @param String $key Name of the attribute
	 * @param String $value Value of the attribute
	 * @see http://www.php.net/manual/es/simplexmlelement.addattribute.php
	 */
	public function addAttribute($key, $value){
		$this->initElement();
		$this->element->addAttribute($key, $value);
	}
	
	private $rows;
	/**
	 * Executes the query
	 */
	public function setRows($rowsToLoad){
		$this->rows = $rowsToLoad;
	}
	public function getRows(){
		return $this->rows;
	}
	protected function loadData(){
		$q = $this->element['query'];
		if ($q != ''){
			$this->rows = null;			
			$prepared = $this->prepareQuery($q);
			$dbo =& JFactory::getDBO();
			$query = $dbo->getQuery(true);
			$dbo->setQuery($prepared);
			$this->rows = $dbo->loadObjectList();
		}
	}
		
	/**
	 * Configures default columns using the data source 
	 * @param Object $record
	 */
	protected function configureColumns($record){
		$aColumns = array();
		$properties = get_object_vars($record);
		foreach($properties as $key=>$value){
			$aColumns[$key] = array('title' => $value);
		}
		$this->setElement('columns', json_encode($aColumns));
	}
	
}

detailtable.php

<?php
/**
 *  Copyright (C) 2013  Pep Lainez
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *  
 */
defined('JPATH_PLATFORM') or die;
require_once JPATH_COMPONENT_ADMINISTRATOR.'/models/fields/dbfield.php';

/**
 * Renders an HTML table fetching data from a SQL sentence
 * The SQL sentence can be parametrized using @var1 ... @varn where varX is the name of a get or post variable.
 * 
 * XML form definition syntax:
 *  <field type="DetailTable" 
 *		sql = "SQL_STATEMENT"
 *		id  = "<an id for the table>"
 *      label = "<a text for the label>"
 *      class = "<class name>"
 *      style = "<CSS inline style>"
 *  	>
 *  		<column title="<column heading>" field="<field to show>" class="<cell style>" style="<CSS inline style>" />  
 *  		<column title="<column heading>" field="<field to show>" class="<cell style>" style="<CSS inline style>" />
 *  		...
 *  </field>
 *  
 *  This field type could be used programmatically too
 *  
 * @author Pep Lainez
 * @package JFormFieldDBFields
 * @since	1.0
 * @see http://docs.joomla.org/Creating_a_custom_form_field_type
 */
class JFormFieldDetailTable extends JFormFieldDBField{
	
	protected $type = "DetailTable";
	
	/**
	 * Prepares the table header
	 * @param Array $columnsDefinition Definition of the table columns
	 * @return string Header of the table
	 */
	private function getTableHeader(){
		
		$result = "<thead>";
		$result .= "<tr>";
		
		foreach ($this->element->children() as $column){
			$class = isset($column['class']) ? ' class="'.$column['class'].'"' : '';
			$style = isset($column['style']) ? ' style="'.$column['style'].'"' : '';
			$result .= sprintf("<th%s%s>%s</th>",$class, $style, JText::_($column['title']) );
		}
		
		$result .= "</tr>";
		$result .= "</thead>";
		return $result;		
	}
		
	/**
	 * Builds a row of the table body 
	 * @param Object $record
	 * @return String
	 */
	protected function getBodyRow($record){
		$result = "";
		foreach($this->element->children() as $column){
			$class = ( isset($column['class'] ) ? ' class="'.$column['class'].'"' : '' );
			$style = ( isset($column['style'] ) ? ' style="'.$column['style'].'"' : '' );
			$result .= sprintf("<td%s%s>%s</td>", $class, $style, $record->$column['field']);
		}
		return $result;
	}
	
	/**
	 * Prepares the table body
	 * @param unknown $records	Records to show
	 * @param unknown $colsDefs	Columns definitions
	 */
	private function getTableBody($records){
		$result = "";
		$i=0;
		foreach($records as $record){
			$result .= "<tr>";
			$result .= $this->getBodyRow($record);
			$result .= "</tr>";
			$i++;
		}
		return $result;		
	}
	
	/**
	 * Prepares the table footer
	 */
	private function getTableFooter(){
		return sprintf("<tfoot><tr><td colspan=\"%s\"></td></tr></tfoot>", $this->element->count());
	}

	/**
	 * Builds an HTML table
	 */
	public function getInput(){
		$html = array();
		$class = ( isset( $this->element['class'] ) ? ' class="'.$this->element['class'].'"' : '' );
		$style = ( isset( $this->element['style'] ) ? ' style="'.$this->element['style'].'"' : '' );
		$records = $this->loadData();
		$html[] = sprintf("<table id=\"%s\"%s%s>", $this->element['id'], $class, $style );
		$html[] = $this->getTableHeader();
		$html[] = $this->getTableBody($records);
		$html[] = $this->getTableFooter();
		$html[] = '</table>';
		return implode("",$html);
	}
	
}

dropdowndb.php

<?php
/**
 * @author	jlainezs
 * @package dbfields
 * @subpackage 
 * @license Licensed under GNU/GPL 3.0
 * @since 1.0
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
defined('JPATH_PLATFORM') or die;
require_once JPATH_COMPONENT_ADMINISTRATOR.'/models/fields/dbfield.php';

/**
 * Renders a combobox
 * @author jlainezs
 * @see http://docs.joomla.org/Creating_a_custom_form_field_type
 */
class JFormFieldDropdownDb extends JFormFieldDBField{
	
	protected $type = "DropdownDb";

	/**
	 * Builds the options of the dropdown
	 * @param Array $records
	 * @return String html options list
	 */
	protected function getDdOptions($records){
		$options = "<option value=\"\">**</option>";
		$fkeyfield = $this->element['key'];
		$flabelField = $this->element['displayvalue'];
		if (count($records)){
			foreach($records as $record){
				$value = $record->$fkeyfield;
				$optLabel = $record->$flabelField;
				$selected = ( $this->value == $value ? " selected=\"selected\"" : "" );					
				$options .= sprintf("<option value=\"%s\"%s>%s</option>",$value, $selected, $optLabel);
			}
		}
		return $options;
	}
	
	/**
	 * Renders a SELECT tag
	 * @return String
	 */
	public function getInput(){
		$html = array();
		$class = ( isset( $this->element['class'] ) ? ' class="'.$this->element['class'].'"' : '' );
		$style = ( isset( $this->element['style'] ) ? ' style="'.$this->element['style'].'"' : '' );
		//$name = ( isset( $this->element['name'] ) ? ' name="'.$this->element['name'].'"' : '' );
		//$id = ( isset( $this->element['id'] ) ? ' id="'.$this->element['id'].'"' : '' );
		$records = $this->loadData();
		$html[] = sprintf("<select id=\"%s\" name=\"%s\"%s%s>", $this->id, $this->name, $class, $style );
		$html[] = $this->getDdOptions($records);
		$html[] = '</select>';
		return implode("",$html);
	} 
}

editable.php

<?php
/**
 * @author	jlainezs
 * @package dbfields
 * @subpackage 
 * @license Licensed under GNU/GPL 3.0
 * @since 1.0
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
defined('JPATH_PLATFORM') or die;
require_once JPATH_COMPONENT_ADMINISTRATOR.'/models/fields/dbfield.php';
 
/**
 * Renders a table with edit capabilities
 * It uses jsTable as a table handling script 
 * Cell elements are JFormField descendants, so we may 
 * use any field type *EXCEPT* JFormFieldEdiTable (this is not controlled!)
 * 
 * @author jlainezs
 * @see http://docs.joomla.org/Creating_a_custom_form_field_type
 * @see http://docs.joomla.org/Retrieving_request_data_using_JInput#Getting_Values_from_a_Specific_Super_Global
 * @see http://mootools.net/forge/p/jstable
 * 
 */
class JFormFieldEdiTable extends JFormFieldDBField{
	
	/**
	 * Field type name
	 * @var String
	 */
	protected $type = "EdiTable";
 	protected $orginalFieldNames = array();

 	/**
 	 * Formats the name
 	 * @param String $originalName
 	 */
 	protected function normalizeFieldName($originalName){
 		$normalizedName = sprintf("%s[%s][]", $this->name, $originalName );
 		return $normalizedName;
 	}
 	
	/**
	 * Gets the column model
	 * @return String the column model
	 */
	public function getColumnModel(){
		$columns = array();
		$n = 0;
		$this->originalFieldNames[] = array();
		foreach($this->element->children() as $field){
			$attributes = $field->attributes();
			$columns[] = new SimpleXMLElement($attributes[0]);
			$attributes = $columns[$n]->attributes();
			$name = (string)$attributes['name'];
			//echo sprintf("%s[%s][]", $this->name, $attributes['name'] )."<br/>";
			$attributes['name'] = $this->normalizeFieldName($name);
			$this->orginalFieldNames[(string)$attributes['name']] = $name;
			$n++;			
		}
		return $columns;
	}
	
	/**
	 * Creates the row action buttons 
	 * @param String $gridId Table identifier
	 * @return String
	 */
	protected function createActionButtons($gridId){
		return	'$e("input",{
					"type"   : "button",
					"value"  : "Del",
					"events" : {
						"click" :	function() {
							var rwid = this.getParent("tr").id;
							var parts = rwid.split("-");
							if (parts.length > 0){
								var rwndx = parts[parts.length-1];
								'.$gridId.'.deleteRowById(parseInt(rwndx));
							}
						}
					}
				})';				
	}
	
	/**
	 * Creates a cell content
	 * @param String $gridId Grid identifier
	 * @param Object $column column definition
	 * @param String $value initial value
	 * @return String HTML Tag for cell content
	 */
	protected function createCellContent($gridId, $column, $value){
		$value = htmlentities($value, ENT_QUOTES, 'UTF-8');
		$attribs = $column[0]->attributes();
		$fld= JFormHelper::loadFieldType($attribs['type'],true);
		if (is_object($fld)){
			$fld->setup($column, $value);
			return $fld->getInput();
		}
		return "";
	}
	
	/**
	 * Gets the primary key definition
	 */
	protected function getPKFieldDefinition($value=""){
		//$a = array('field'=>$this->getKeyFieldName(), 'dataType'=>'hidden');
		$pkfld = new SimpleXMLElement("<field type=\"hidden\" name=\"".$this->getKeyFieldName()."\" value=\"".$value."\" />");
		return $this->createCellContent("", $pkfld, $value);
	}
	
	/**
	 * Creates the table columns.
	 * 		jsTable requires to create the columns before adding rows.
	 * @param String $gridId the grid
	 * @param String $html HTML *REFERENCE* to add the javascript
	 * @return Array:String columns definitions 
	 */
	protected function createColumns($gridId, &$html){
		$cols = $this->getColumnModel();
		$newRowDeclaration = array();
		$value = "";
		
		foreach($cols as $column){
			$newRowDeclaration[] = $this->createCellContent($gridId, $column, $value);
			$attribs = $column->attributes();
			$html .= "  $gridId.addColumn( '".$attribs['label']."' ); "; 
		}
		$newRowDeclaration[0] = $this->getPKFieldDefinition().$newRowDeclaration[0];
		return $newRowDeclaration;		
	}
	
	/**
	 * Initializes the data of the grid
	 * @param String $gridId Grid identifier
	 * @return String Javascript fragment which adds existing rows into the table
	 */
	protected function initializeTable($gridId){
		$columns = $this->getColumnModel();
		$deleteButton =  $this->createActionButtons($gridId);
		if (count($this->getRows())){
			foreach($this->getRows() as $row){
				$pkcol = $this->getPKFieldDefinition();
				$content = array();
				foreach($columns as $column){
					$attribs = $column->attributes();
					$fld = $this->orginalFieldNames[(string)$attribs['name']];
					if ($fld != ''){
						$content[] = $this->createCellContent($gridId, $column, $row->$fld);
					}
				}
				$fld = $this->getKeyFieldName();
				$content[0] = $this->getPKFieldDefinition($row->$fld).$content[0]; 
				$jsContent = "'".implode("','", $content) . "'";
				$html .= $gridId.".addRow( $jsContent, $deleteButton);\r\n";
			}
		}
		return $html;
	}
	
	/**
	 * Prepares the table markup
	 * @param String $gridId Table identifier
	 */
	protected function createTable($gridId){
		$html = "
		<script type='text/javascript'>
		window.addEvent('load', function(){
			var $gridId = new jsTable('$gridId');
		";
		
		$newRowDeclaration = $this->createColumns($gridId, $html);
		$html .= "	$gridId.addColumn('Acciones');";
		$deleteButton = $this->createActionButtons($gridId);
		$html .= "
			document.id(\"addRowBtn-$gridId\")
				.addEvent(\"click\", function(){
					var clls = new Array();
					var fldId = new Date().getTime();
				 ";
		$i=0;
		$newcellscontent="";
		foreach($newRowDeclaration as $rowDec){
			$html .= "					clls[$i] = '$rowDec';\r\n";
			$i++;
			$newcellscontent .= ($newcellscontent != '' ? ',' : '').'""'; 
		}
		$html .= "	
					$gridId.addRow( $newcellscontent, $deleteButton );
					for(i=0;i<$gridId.columnCount()-1;i++){
						$gridId.setCell($gridId.rowCount()-1, i, clls[i]);
					}
				}); // addEvent
				$gridId.setEmptyMessage('No hay datos disponibles.',true);
				";

		$html .= $this->initializeTable($gridId);
			
		$html .= "
		}); // window.addEvent
		</script>
		<div class=\"toolbar\">
			<input type=\"button\" value=\"Añadir entrada\" id=\"addRowBtn-$gridId\"/>
		</div>		
		";
		return $html;
	}
	
	/**
	 * Sets the name of the control
	 * @param String $name
	 */
	public function setName($name){
		$this->name = $name;
	}
	
	/**
	 * @var String keyfield Key field name holder
	 */
	private $keyFieldName= "";
	
	/**
	 * Sets the keyfield name
	 * @param String $fieldName the key field name
	 */
	public function setKeyFieldName($fieldName){
		$this->keyFieldName = $fieldName;
	}
	
	/**
	 * Gets the key field name
	 * @return String
	 */
	public function getKeyFieldName(){
		return $this->normalizeFieldName( $this->keyFieldName );
	}
	
	/**
	 * Generates a grid
	 * 	whit edit capabilities
	 */	
	public function getInput(){
		
		$doc =& JFactory::getDocument();
		$doc->addScript(JURI::base().'administrator/components/com_competences/models/fields/editableassets/jsTable.js');
		$doc->addStyleSheet(JURI::base().'administrator/components/com_competences/models/fields/editableassets/jstable-default.css');
		
		if (!count($this->getRows()))
			$this->loadData();

		$gridId = "tbl".md5(uniqid(time));
		$htmlGrid = $this->createTable($gridId);
		return
			'<div class="editable-wrapper">'
				.$htmlGrid
				.'<div id="'.$gridId.'" class="editable"></div>'
			.'</div>';
	}
}

sample.php

<?php 
defined('_JEXEC') or die('Restricted access');
JHTML::_('behavior.mootools');
?>
<script type="text/javascript">
function save(){
	document.id('frmtask').value="profile.savelaboralexperience";
	document.id('frm-laboral-experience').submit();
}
</script>

<form id = "frm-laboral-experience" method = "post">
	<h3><?php echo JText::_('COM_COMPETENCES_PROFILE_PROFESSIONALEXPERIENCE')?></h3>
	<div id="list-experience"></div>
	<?php
		JFormHelper::addFieldPath(JPATH_COMPONENT_ADMINISTRATOR.'/models/fields');
		$expLbl = JFormHelper::loadFieldType('EdiTable',false);
		$expLbl->setColumns( $this->columns );
		$expLbl->setKeyFieldName('id');
		$expLbl->setName('laboralExperience');

		$cols = array();
		$cols[] = array('title'=>'Durada', 'dataType' => 'string',  'maxlength'=>4, 'size' => 4, 'field'=>'contractduration');
		$cols[] = array('title'=>'Centre/Organisme', 'dataType' => 'string', 'maxlength'=>30, 'size' => 25,'field'=>'company');
		$cols[] = array('title'=>'Sector productiu', 'dataType' => 'string', 'maxlength'=>4, 'size' => 4,'field'=>'idsector');
		$cols[] = array('title'=>'Lloc de treball', 'dataType' => 'string', 'maxlength'=>30, 'size' => 30,'field'=>'occupation');
		$cols[] = array('title'=>'Tasques', 'dataType' => 'text',  'maxlength'=>30, 'size' => 30,'field'=>'tasks');
		$cols[] = array('title'=>'Acciones', 'dataType' => 'actions', 'maxlength'=>0, 'size'=>0);
		
		$expLbl->setRows($cols);
		
		echo $expLbl->getInput(); 
	?>
	<div class="toolbar">
		<input type="button" value="Guardar" onclick="javascript:save();" />
		<input id="frmtask" type="hidden" name="task" value = "" />
		<input type="hidden" name="basic[iduser]" value="<?php echo $this->profile->iduser?>" />
		<input type="hidden" name="basic[id]" value="<?php echo $this->profile->id?>" />
	</div>
</form>

This code is under development, so take it as a proof of concept.
In fact, it has security issues as it can suffers an SQL injection. You are warned... use at your own risk.