mybatis：updatebyexample与updateByExampleSelective

　　MyBatis，通常逆向工程工具生成接口和xml映射文件用于简单的单表操作。

　　有两个方法： updateByExample 和 updateByExampleSelective  ，作用是对数据库进行更新操作。（insert和insertSelective也一球样！）

注意：1，区别在于后者比前者多了动态标签<if test="xxx != null">的判断，有空值，不进行插入操作。

　　　2，updateByExampleSelective可以不按主键更新，条件中可以不包含主键；updateByExample条件中必须包含主键。

updateByExample：

 <update id="updateByExample" parameterType="map" >
    update tb_item
    set id = #{record.id,jdbcType=BIGINT},
      title = #{record.title,jdbcType=VARCHAR},
      sell_point = #{record.sellPoint,jdbcType=VARCHAR},
      price = #{record.price,jdbcType=BIGINT},
      num = #{record.num,jdbcType=INTEGER},
      barcode = #{record.barcode,jdbcType=VARCHAR},
      image = #{record.image,jdbcType=VARCHAR},
      cid = #{record.cid,jdbcType=BIGINT},
      status = #{record.status,jdbcType=TINYINT},
      created = #{record.created,jdbcType=TIMESTAMP},
      updated = #{record.updated,jdbcType=TIMESTAMP}
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
</update>

updateByExampleSelective：

<update id="updateByExampleSelective" parameterType="map" >
    update tb_item
    <set >
      <if test="record.id != null" >
        id = #{record.id,jdbcType=BIGINT},
      </if>
      <if test="record.title != null" >
        title = #{record.title,jdbcType=VARCHAR},
      </if>
      <if test="record.sellPoint != null" >
        sell_point = #{record.sellPoint,jdbcType=VARCHAR},
      </if>
      <if test="record.price != null" >
        price = #{record.price,jdbcType=BIGINT},
      </if>
      <if test="record.num != null" >
        num = #{record.num,jdbcType=INTEGER},
      </if>
      <if test="record.barcode != null" >
        barcode = #{record.barcode,jdbcType=VARCHAR},
      </if>
      <if test="record.image != null" >
        image = #{record.image,jdbcType=VARCHAR},
      </if>
      <if test="record.cid != null" >
        cid = #{record.cid,jdbcType=BIGINT},
      </if>
      <if test="record.status != null" >
        status = #{record.status,jdbcType=TINYINT},
      </if>
      <if test="record.created != null" >
        created = #{record.created,jdbcType=TIMESTAMP},
      </if>
      <if test="record.updated != null" >
        updated = #{record.updated,jdbcType=TIMESTAMP},
      </if>
    </set>
    <if test="_parameter != null" >
      <include refid="Update_By_Example_Where_Clause" />
    </if>
  </update>

 UpdateByExampleSelectivce使用：

　　ProLogExample example = new ProLogExample();
　　example.createCriteria().andNameEqualTo(xiaoming.getName());
　　proLogMapper.updateByExampleSelective(xiaoming, example);