<!--    动态 SQL-->

    <select id="queryBlogIF" parameterType="map" resultType="B">

        select * from blog where 1=1

        <!-- 如果 title 不为 null 则拼接以下语句 -->

        <if test="title != null">

            and title = #{title}

        </if>

        <!-- 如果 author 不为 null 则拼接以下语句 -->

        <if test="author != null">

            and author = #{author}

        </if>

    </select>