Statement returned more than one row, where no more than one was expected

Statement returned more than one row, where no more than one was expected

<resultMap id="Student" type="org.mybatis.example.Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
</resultMap>

<resultMap id="InfoResult" type="org.mybatis.example.Student">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <result property="age" column="age" />
    <association property="personalInfo" column="id" javaType="org.mybatis.example.PersonalInfo"
        select="getPersonalInfo"/>
</resultMap>

<select id="getStudentInfo" parameterType="int" resultMap="InfoResult">
    select * from student where id &lt; #{id}
</select>

<resultMap id="PersonalInfo" type="org.mybatis.example.PersonalInfo">
    <id property="id" column="id" />
    <result property="height" column="height" />
    <result property="weight" column="weight" />
</resultMap>

<!-- 选择学生的个人信息 -->
<select id="getPersonalInfo" parameterType="int" resultMap="PersonalInfo">
    select * from personal_info where id &lt; #{id}
</select>

下面是测试文件的片段

statement = "org.mybatis.mapping.studentMapper.getStudentInfo";
List<Student> students;
students = session.selectList(statement,2);
System.out.println(students.size());
for(int i=0; i<students.size(); i++) {
	System.out.println(students.get(i).toString());

输入2时，返回结果为：

1

Student [name=张三, age=20, id=1, personalInfo=null]

输入3时，返回结果为：

2

Student [name=张三, age=20, id=1, personalInfo=null]

Student [name=李四, age=19, id=2, personalInfo=PersonalInfo [id=1, height=172, weight=67.2]]

输入4时，返回结果为

### Error querying database.  Cause: org.apache.ibatis.executor.ExecutorException: Statement returned more than one row, where no more than one was expected.
### The error may exist in org/mybatis/mapping/studentMapper.xml
### The error may involve org.mybatis.mapping.studentMapper.getPersonalInfo
### The error occurred while handling results
### SQL: select * from personal_info where id < ?

我也是服了网上的这群哥哥们，所有的嵌套查询的例子都是输一个id，然后用`id = #{id}`这种条件来查询的，就没有其他的情况么？？？