<insert id="insertSelective" keyColumn="id" keyProperty="id" parameterType="com.second.hand.trading.server.model.IdleItemModel" useGeneratedKeys="true">
insert into sh_idle_item
<trim prefix="(" suffix=")" suffixOverrides=",">
<if test="idleName != null">
idle_name,
</if>
<if test="idleDetails != null">
idle_details,
</if>
<if test="pictureList != null">
picture_list,
</if>
<if test="idlePrice != null">
idle_price,
</if>
<if test="idlePlace != null">
idle_place,
</if>
<if test="idleLabel != null">
idle_label,
</if>
<if test="releaseTime != null">
release_time,
</if>
<if test="idleStatus != null">
idle_status,
</if>
<if test="userId != null">
user_id,
</if>
</trim>
<trim prefix="values (" suffix=")" suffixOverrides=",">
<if test="idleName != null">
#{idleName,jdbcType=VARCHAR},
</if>
<if test="idleDetails != null">
#{idleDetails,jdbcType=VARCHAR},
</if>
<if test="pictureList != null">
#{pictureList,jdbcType=VARCHAR},
</if>
<if test="idlePrice != null">
#{idlePrice,jdbcType=DECIMAL},
</if>
<if test="idlePlace != null">
#{idlePlace,jdbcType=VARCHAR},
</if>
<if test="idleLabel != null">
#{idleLabel,jdbcType=INTEGER},
</if>
<if test="releaseTime != null">
#{releaseTime,jdbcType=TIMESTAMP},
</if>
<if test="idleStatus != null">
#{idleStatus,jdbcType=TINYINT},
</if>
<if test="userId != null">
#{userId,jdbcType=BIGINT},
</if>
</trim>
这个mybaits的是插入一个商品的信息,为什么要重复两次,两个trim标签的作用是什么?