mybatis的where和if标签配合使用

<select id="getCountByPageInfo" parameterType="com.dabai.mytwo.entity.PageInfo">
        select count(*)
        from forum
        <where>
            <if test="institutionid!=null">
                institution_id=#{institution_id}
            </if>
            <if test="userid!=null">
                author_id=#{userid}
            </if>
        </where>
    </select>

public class PageInfo implements Serializable {

    private static final long serialVersionUID = 1L;
    private Integer totalcount;
    private Integer currentpage;
    private Integer totalpage;
    private Integer pagesize;
    private Long institutionid;
    private Long forumid;
    private String userid;
    private String ids;
    private String code;
    //get  set方法
}