ibatis isNotNull isNotEmptyの違い

	<!--          -->
	<select id="getUserPrivacyPage" parameterClass="map" resultClass="hashmap">
	select uid,`key`,`value` from ts_user_privacy
		<dynamic prepend="WHERE">
			<isNotEmpty prepend="AND" property="uid">
			uid=#uid#
			</isNotEmpty>
			<isNotEmpty prepend="AND" property="key">
			key=#key#
			</isNotEmpty>
			<isNotEmpty prepend="AND" property="value">
			value=#value#
			</isNotEmpty>
		</dynamic>
		<isNotEmpty prepend="order by " property="_order">
		$_order$
		</isNotEmpty>
		<isNotEmpty prepend="limit " property="_limit">
		 #_limit._pstart#,#_limit._psize#
		</isNotEmpty>
	</select>

	<update id="updateUserPrivacy"  parameterClass="map">
	update ts_user_privacy 
		<dynamic  prepend="set">
			<isNotNull prepend=","  property="value"> `value`=#value# </isNotNull>
		</dynamic>
		where uid=#uid# AND `key`=#key#
	</update>