mybatis에서 selectkey해서 insert를 해주려고 하는데요 selectkey해서 no row returned되면 그냥 공백으로 넣어주고 싶은데 어떻게 해야하나요??

예)

<insert id="insertStudent" parameterType="map">

<selectKey keyProperties="SCHOOL_NAME" resultType="map" order="BEFORE">

SELECT SCHOOL_NAME FROM SCHOOL WHERE SCHOOL_CODE=#{SCHOOL_CODE}

</selectKey>

INSERT INTO STUDENT ( ST_NUM, ST_NAME, ~~~, SCHOOL_CODE, SCHOOL_NAME )

VALUES (#{ST_NUM}, #{ST_NAME}, ~~~, #{SCHOOL_CODE}, #{SCHOOL_NAME})

</insert>

SCHOOL_NAME이 NULL로 반환되면 그냥 공백으로 넣는 걸 하고싶어요..!