Suggest that "resultMap" add a "ignoreNotExists" attribute:

 

<resultMap id="xresult" class="xclass" ignoreNotExists="true">

         <result property="a" column="a" />

         <result property="b" column="b" />

         <result property="c" column="c" />

         <result property="d" column="d" />

</resultMap>

 

<select id="s1" resultMap="xresult">

         select a, b from x

</select>

 

<select id="s2" resultMap="xresult">

         select a, b, c from x

</select>