Mybatis培训教程

5. 解决字段名与实体类属性名不相同的冲突

5.1. 准备表和数据：

CREATE TABLE orders( order_id INT PRIMARY KEY AUTO_INCREMENT, order_no VARCHAR(20), order_price FLOAT ); INSERT INTO orders(order_no, order_price) VALUES('aaaa', 23); INSERT INTO orders(order_no, order_price) VALUES('bbbb', 33); INSERT INTO orders(order_no, order_price) VALUES('cccc', 22);

5.2. 定义实体类：

public class Order { private int id; private String orderNo; private float price; }

5.3. 实现getOrderById(id)的查询：

方式一: 通过在sql语句中定义别名 <select id="selectOrder" parameterType="int" resultType="_Order"> select order_id id, order_no orderNo,order_price price from orders where order_id=#{id} </select>   方式二: 通过<resultMap> <select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap"> select * from orders where order_id=#{id} </select>   <resultMap type="_Order" id="orderResultMap"> <id property="id" column="order_id"/> <result property="orderNo" column="order_no"/> <result property="price" column="order_price"/> </resultMap>

 

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

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：

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

<!-- 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

@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); }

 

 

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