mybatis的where标签

<trim prefix="where" prefixOverrides="and">
       <if test="id!=null">
           id=#{id}
       </if>
       <if test="username!=null">
           and username=#{username}
       </if>
</trim>
等同于

<where>
       <if test="id!=null">
           id=#{id}
       </if>
       <if test="username!=null">
           and username=#{username}
       </if>
</where>