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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.tid = t.tid AND c.cid=#{id}
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 c,teacher t WHERE c.tid = t.tid AND c.cid=#{id}
SELECT t.tid id, t.tname name FROM teacher t WHERE t.tid = #{id} //使用上面查到的tid
2种都用到了resultMap和association（关联），区别：
方式1中的association中，用到javaType，代表关联的实体类型
方式2中的association中，不用javaType，但是用到column（代表第一条sql查询的结果的外键作为第二条sql的查询条件）和select（第二条sql语句）
实例：
有班级class[id, name, teacher]，对应的老师teacher[id, name]，它们的关系是一对一。
插入表数据：
--teacher表 INSERT INTO `mybatis`.`teacher`(`tid`,`tname`) VALUES ( NULL,‘老师A‘); INSERT INTO `mybatis`.`teacher`(`tid`,`tname`) VALUES ( NULL,‘老师B‘);
--class表 INSERT INTO `mybatis`.`class`(`cid`,`cname`,`tid`) VALUES ( NULL,‘一年级‘,‘1‘); INSERT INTO `mybatis`.`class`(`cid`,`cname`,`tid`) VALUES ( NULL,‘二年级‘,‘2‘);
Java实体类：
public class Teacher { private int id; private String name; public Teacher() { super(); } public Teacher(int id, String name) { super(); this.id = id; this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Teacher [id=" + id + ", name=" + name + "]"; } }
public class Clazz { private int id; private String name; private Teacher teacher; public Clazz(int id, String name, Teacher teacher) { super(); this.id = id; this.name = name; this.teacher = teacher; } public Clazz() { super(); } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } @Override public String toString() { return "Clazz [id=" + id + ", name=" + name + ", teacher=" + teacher + "]"; } }
SELECT * FROM class c,teacher t WHERE c.tid = t.tid AND c.cid=#{id}
teacherMapper.xml:
<!-- 查询班级及其老师信息 --> <select id="getClazz" parameterType="int" resultMap="getClazzMap"> SELECT * FROM class c,teacher t WHERE c.tid = t.tid AND c.cid=#{id} </select> <resultMap type="Clazz" id="getClazzMap"> <id property="id" column="cid"/> <result property="name" column="cname"/> <!-- 关联班级对应的teacher --> <association property="teacher" javaType="Teacher"> <id property="id" column="tid"/> <result property="name" column="tname"/> </association> </resultMap>
Java测试类：
/** * 方式一：嵌套结果 * @author 魅力_小生 * */ @Test public void getClazz1(){ //创建session，设置事务为true SqlSession session = MyBatisUtil.getSessionFactory().openSession(true); String statement = "com.mlxs.mybatis.test4.clazzMapper.getClazz"; Clazz clazz = session.selectOne(statement, "2"); System.out.println("clazz--->"+clazz); session.close(); }
结果：
clazz--->Clazz [id=2, name=二年级, teacher=Teacher [id=2, name=老师B]]
SELECT * FROM class c,teacher t WHERE c.tid = t.tid AND c.cid=#{id}
SELECT t.tid id, t.tname name FROM teacher t WHERE t.tid = #{id} //使用上面查到的tid
teacherMapper.xml:
<!-- 一对一 关联查询： 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 c,teacher t WHERE c.tid = t.tid AND c.cid=#{id} SELECT t.tid id, t.tname name FROM teacher t WHERE t.tid = #{id} //使用上面查到的tid --> <!-- 查询班级及其老师信息 --> <select id="getClazz2" parameterType="int" resultMap="getClazzMap2"> SELECT * FROM class c,teacher t WHERE c.tid = t.tid AND c.cid=#{id} </select> <select id="getTeacher" parameterType="int" resultType="Teacher"> SELECT t.tid id, t.tname name FROM teacher t WHERE t.tid = #{id} </select> <resultMap type="Clazz" id="getClazzMap2"> <id property="id" column="cid"/> <result property="name" column="cname"/> <!-- 关联班级对应的teacher，将查询class表返回的tid参数传给getTeacher，也就是getTeacher中的#{id} --> <association property="teacher" column="tid" select="getTeacher"/> </resultMap>
Java测试类：
/** * 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 c,teacher t WHERE c.tid = t.tid AND c.cid=#{id} SELECT t.tid id, t.tname name FROM teacher t WHERE t.tid = #{id} //使用上面查到的tid * @author 魅力_小生 * */ @Test public void getClazz2(){ //创建session，设置事务为true SqlSession session = MyBatisUtil.getSessionFactory().openSession(true); String statement = "com.mlxs.mybatis.test4.clazzMapper.getClazz2"; Clazz clazz = session.selectOne(statement, "2"); System.out.println("clazz--->"+clazz); session.close(); }
结果：
clazz--->Clazz [id=1, name=一年级, teacher=Teacher [id=1, name=老师A]]
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原文地址：http://www.cnblogs.com/yzlpersonal/p/5208542.html
