• prefix trim标签内sql语句加前缀
• suffix 加后缀
• prefixOverrides 去除多余的前缀
• suffixOverrides 去除多余的后缀

<insert id="insert" parameterType="com.tortuousroad.groupon.cart.entity.Cart">
 insert into cart
 <trim prefix="(" suffix=")" suffixOverrides=",">
     <if test="id != null">
         id,
     </if>
     <if test="userId != null">
         user_id,
     </if>
     <if test="count != null">
         count,
     </if>
     <if test="createTime != null">
         create_time,
     </if>
     <if test="updateTime != null">
         update_time,
     </if>
 </trim>
 <trim prefix="values (" suffix=")" suffixOverrides=",">
     <if test="id != null">
         #{id,jdbcType=BIGINT},
     </if>
     <if test="userId != null">
         #{userId,jdbcType=BIGINT},
     </if>
     <if test="count != null">
         #{count,jdbcType=INTEGER},
     </if>
     <if test="createTime != null">
         #{createTime,jdbcType=TIMESTAMP},
     </if>
     <if test="updateTime != null">
         #{updateTime,jdbcType=TIMESTAMP},
     </if>
 </trim>
</insert>

<update id="update" parameterType="com.bank.vo.Pledge">
	update bank_pledge
	<trim prefix="set" suffixOverrides=",">
		<if test="certificate!=null">
			certificate=#{certificate,jdbcType=VARCHAR},
		</if>
	</trim>
	where id=#{id}
</update>