mybatis实现关联表查询

一对一关联

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
-->
<mapper namespace="me.gacl.mapping.classMapper">

<!--
根据班级id查询班级信息(带老师的信息)
##1. 联表查询
SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;

##2. 执行两次查询
SELECT * FROM class WHERE c_id=1; //teacher_id=1
SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
-->

<!--
方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集
封装联表查询的数据(去除重复的数据)
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
-->
<select id="getClass" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" javaType="me.gacl.domain.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
</resultMap>

<!--
方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
-->
<select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select * from class where c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" select="getTeacher"/>
</resultMap>

<select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>

</mapper>

一对多关联

<!--
根据classId查询对应的班级信息,包括学生,老师
-->
<!--
方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1
-->
<select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
</select>
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<!-- ofType指定students集合中的对象类型 -->
<collection property="students" ofType="me.gacl.domain.Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</collection>
</resultMap>

<!--
方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值
-->
<select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
select * from class where c_id=#{id}
</select>
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
<collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
</resultMap>

<select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>

<select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
</select>