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>
