Mybatis培训教程
6.实现关联表查询
6.1. 一对一关联
1). 提出需求
根据班级id查询班级信息(带老师的信息)
2). 创建表和数据
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CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20) ); CREATE TABLE class( c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(20), teacher_id INT ); ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);
INSERT INTO teacher(t_name) VALUES('LS1'); INSERT INTO teacher(t_name) VALUES('LS2');
INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1); INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2); |
3). 定义实体类：
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public class Teacher { private int id; private String name; } public class Classes { private int id; private String name; private Teacher teacher; } |
4). 定义sql映射文件ClassMapper.xml
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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.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> <resultMap type="_Classes" id="ClassResultMap"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_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> <resultMap type="_Classes" id="ClassResultMap2"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher"> </association> </resultMap>
<select id="getTeacher" parameterType="int" resultType="_Teacher"> SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select> |
5). 测试
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@Test public void testOO() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass", 1); System.out.println(c); }
@Test public void testOO2() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass2", 1); System.out.println(c); } |
6.2. 一对多关联
1). 提出需求
根据classId查询对应的班级信息,包括学生,老师
2). 创建表和数据：
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CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT ); INSERT INTO student(s_name, class_id) VALUES('xs_A', 1); INSERT INTO student(s_name, class_id) VALUES('xs_B', 1); INSERT INTO student(s_name, class_id) VALUES('xs_C', 1); INSERT INTO student(s_name, class_id) VALUES('xs_D', 2); INSERT INTO student(s_name, class_id) VALUES('xs_E', 2); INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);
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3). 定义实体类：
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public class Student { private int id; private String name; }
public class Classes { private int id; private String name; private Teacher teacher; private List<Student> students; } |
4). 定义sql映射文件ClassMapper.xml
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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 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="_Classes" id="ClassResultMap3"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_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="_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="_Classes" id="ClassResultMap4"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher2"></association> <collection property="students" ofType="_Student" column="c_id" select="getStudent"></collection> </resultMap>
<select id="getTeacher2" parameterType="int" resultType="_Teacher"> SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select>
<select id="getStudent" parameterType="int" resultType="_Student"> SELECT s_id id, s_name name FROM student WHERE class_id=#{id} </select> |
5). 测试：
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@Test public void testOM() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass3", 1); System.out.println(c); }
@Test public void testOM2() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass4", 1); System.out.println(c); }
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