我们先来看看这两张表
表名：tbl_Student
字段：ID,NAME,CLASS_ID
表名：tbl_Class
字段：ID，NAME
很简单的一个逻辑，班级下边有很多学生。我们先对学生表做简单的查询。
Model
public class Student
{
private String id;
private String name;
private Stirng birth;
private String class_id;
.......................
}
StudentMapper.xml
<typeAlias alias="StudentClass" type="model.Student"></typeAlias>
<sql id="select">
<![CDATA[
select ID,NAME,CLASS_ID
from tbl_Student
]]>
</sql>
<resultMap id="StudentMap" class="StudentClass">
<result property="id" column="ID" />
<result property="name" column="NAME" />
<result property="class_id" column="CLASS_ID" />
</resultMap>
<select id="selectAll" parameterClass="java.util.Map" resultMap="StudentMap">
<include refid="select" />
</select>
但是我想加入班级名称在里面呢？这个时候Student就不够用了，我们需要用dto了。
Dto
public class StudentDto
{
private String id;
private String name;
private Stirng birth;
private String class_id;
private String class_name;
......................
}
在原有的StudentMapper.xml文件中加入一下内容：
<typeAlias alias="StudentDtoClass" type="dto.StudentDto"></typeAlias>
<resultMap id="StudentDtoMap" class="StudentDtoClass">
<result property="id" column="ID" />
<result property="name" column="NAME" />
<result property="class_id" column="CLASS_ID" />
<result property="class_name" column="CLASS_NAME" />
</resultMap>
<sql id="select2">
<![CDATA[
select s.ID as id,s.NAME as name,s.CLASS_ID as class_id,c.name as class_name
from tbl_Student s,tbl_Class c
where s.class_id = c.id
]]>
</sql>
<select id="selectAll2" parameterClass="java.util.Map" resultMap="StudentDtoMap">
<include refid="select2" />
</select>
一定要注意写as后面的值，这样才能一一对应起来，否则会报错：未指定的列。