mysql auto-inc를 쓸때, auto-inc된 값이 필요할 때 selectKey 쓰는 방식
https://yookeun.github.io/java/2014/07/11/mybatis-selectkey/
<insert id="insertBoard"
parameterType="{packagePath}.BoardVO"
useGeneratedKeys="true"
keyProperty="boardNo">
...
</insert>