mybatis动态更新 set标签 ，自动去掉set语句 尾部 多余的逗号 


	<update id="update2" parameterType="demo.data.UserDO">
		UPDATE t_user
		<set>
			updatetime=now(),
			<if test="birth != null">birth = #{birth},</if>
			<if test="height != null">height = #{height},</if>
			<if test="weight != null">weight = #{weight},</if>
		</set>
		WHERE
		id = #{id}
	</update>


height weight 不为 null 的情况  ，动态生成 sql 

update t_user set updatetime=now(), height=?,weight=? where id=?

weight=? 后面的逗号会被自动去掉

例子代码
https://gitee.com/dyyx/springboothello/blob/master/src/main/resources/mapper/UserMapper.xml