当两个实体具有1对1的对应关系时，可以使用One-To-One的进行映射关联查询
One-To-One示例数据
以学生表Student和地址信息表为例，每个学生都有都有1个唯一的地址(现实中，这种对应关系是不合适的，因为人和地址是多对一的关系)，这里只是演示目的
学生表
CREATE TABLE STUDENTS ( STUD_ID INT(11) NOT NULL AUTO_INCREMENT, NAME VARCHAR(50) NOT NULL, EMAIL VARCHAR(50) NOT NULL, PHONE VARCHAR(15) DEFAULT NULL, DOB DATE DEFAULT NULL, GENDER VARCHAR(6) DEFAULT NULL, BIO LONGTEXT DEFAULT NULL, PIC BLOB DEFAULT NULL, ADDR_ID INT(11) DEFAULT NULL, PRIMARY KEY (STUD_ID), UNIQUE KEY UK_EMAIL (EMAIL), CONSTRAINT FK_STUDENTS_ADDR FOREIGN KEY (ADDR_ID) REFERENCES ADDRESSES (ADDR_ID) ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;
地址表：
CREATE TABLE ADDRESSES ( ADDR_ID INT(11) NOT NULL AUTO_INCREMENT, STREET VARCHAR(50) NOT NULL, CITY VARCHAR(50) NOT NULL, STATE VARCHAR(50) NOT NULL, ZIP VARCHAR(10) DEFAULT NULL, COUNTRY VARCHAR(50) NOT NULL, PRIMARY KEY (ADDR_ID) ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;
样例数据：
INSERT INTO ADDRESSES (ADDR_ID,STREET,CITY,STATE,ZIP,COUNTRY) VALUES (1,'4891 Pacific Hwy','San Diego','CA','92110','San Diego'), (2,'2400 N Jefferson St','Perry','FL','32347','Taylor'), (3,'710 N Cable Rd','Lima','OH','45825','Allen'), (4,'5108 W Gore Blvd','Lawton','OK','32365','Comanche'); -- Sample data for table STUDENTS INSERT INTO STUDENTS (STUD_ID,NAME,EMAIL,PHONE,DOB,BIO,PIC,ADDR_ID) VALUES (1,'Timothy','[email protected]','123-123-1234','1988-04-25',NULL,NULL,3), (2,'Douglas','[email protected]','789-456-1234','1990-08-15',NULL,NULL,4);
从上面的建表的sql中，可以看到，STUDENTS表通过外键ADDR_ID与ADDRESSES表建立1:1的映射关系
建立Address-Mapper.xml映射文件
<?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">
<mapper namespace="com.mybatis3.mappers.AddressMapper">
<!--Address是在MyBatis主配置文件中定义的类型别名-->
<!--AddressResult这个resultMap对ADDRESSES表进行了SQL和Model的映射-->
<resultMap type="Address" id="AddressResult">
<id property="addrId" column="addr_id"/>
<result property="street" column="street"/>
<result property="city" column="city"/>
<result property="state" column="state"/>
<result property="zip" column="zip"/>
<result property="country" column="country"/>
</resultMap>
<!--查询指定的Address-->
<select id="selectAddressById" parameterType="int" resultMap="AddressResult">
select * from addresses where addr_id=#{addrId}
</select>
</mapper>
建立Student-Mapper.xml映射文件
<mapper namespace="com.mybatis3.mappers.StudentMapper">
<resultMap type="Student" id="StudentResult">
<id property="studId" column="stud_id"/>
<result property="name" column="name" />
<result property="email" column="email"/>
<result property="phone" column="phone"/>
</resultMap>
<resultMap type="Student" id="StudentWithAddressExtResult" extends="StudentResult">
<result property="address.addrId" column="addr_id"/>
<result property="address.street" column="street"/>
<result property="address.city" column="city"/>
<result property="address.state" column="state"/>
<result property="address.zip" column="zip"/>
<result property="address.country" column="country"/>
</resultMap>
<resultMap type="Student" id="StudentWithAddressNestedSelect">
<id property="studId" column="stud_id"/>
<result property="name" column="name"/>
<result property="email" column="email"/>
<association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/>
</resultMap>
<resultMap type="Student" id="StudentWithAddressNestedResultMap">
<id property="studId" column="stud_id"/>
<result property="name" column="name"/>
<result property="email" column="email"/>
<association property="address" javaType="Address">
<id property="addrId" column="addr_id"/>
<result property="street" column="street"/>
<result property="city" column="city"/>
<result property="state" column="state"/>
<result property="zip" column="zip"/>
<result property="country" column="country"/>
</association>
</resultMap>
<select id="findAllStudents" resultMap="StudentResult">
select * from Students
</select>
<select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, STREET, CITY, STATE, ZIP, COUNTRY
FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
WHERE STUD_ID=#{studId}
</select>
<select id="findStudentWithAddressNestedSelect" parameterType="int" resultMap="StudentWithAddressNestedSelect">
select * from Students where stud_id=#{studId}
</select>
<select id="findStudentWithAddressNestedResultMap" parameterType="int" resultMap="StudentWithAddressNestedResultMap">
select stud_id, name, email,phone, a.addr_id, street, city, state, zip, country
FROM students s left outer join addresses a on s.addr_id=a.addr_id
where stud_id=#{studId}
</select>
</mapper>
三种One-To-One映射的写法
从Student-Mapper.xml中，可以看到，One-To-One的映射关系可以有三种写法
findStudentWithAddressExtResult findStudentWithAddressNestedSelect findStudentWithAddressNestedResultMap
One-To-One映射：使用resultMap扩展(不推荐，了解即可)
1. resultMap定义
<resultMap type="Student" id="StudentResult"> <!--定义Student表和Student之间的映射关系--> <id property="studId" column="stud_id"/> <result property="name" column="name" /> <result property="email" column="email"/> <result property="phone" column="phone"/> </resultMap> <resultMap type="Student" id="StudentWithAddressExtResult" extends="StudentResult"> <!--扩展Student表和Student之间的映射关系--> <result property="address.addrId" column="addr_id"/> <!--property的写法使用.语法，类似OGNL表达式语言--> <result property="address.street" column="street"/> <!--column从哪里来的？这是在SQL语句中定义--> <result property="address.city" column="city"/> <result property="address.state" column="state"/> <result property="address.zip" column="zip"/> <result property="address.country" column="country"/> </resultMap>
2.SQL-Mapping定义
<select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, STREET, CITY, STATE, ZIP, COUNTRY
FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
WHERE STUD_ID=#{studId}
</select>
在SQL-Mapping中定义了StudentWithAddressExtResult定义的列名，这里看出，列名的定义，可以不关心是来自哪个表，MyBatis只是将得到的列名与属性名进行匹配，这样就有个问题：假如Student表和Address表都有一个相同的列name，如下所示，那么如果为两个NAME设置对应的值？
<select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, A.NAME, STREET, CITY, STATE, ZIP, COUNTRY
FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
WHERE STUD_ID=#{studId}
</select>
One-To-One映射：使用association + select属性（直观简洁，推荐用法）
1.resultMap定义
<resultMap type="Student" id="StudentWithAddressNestedSelect"> <id property="studId" column="stud_id"/> <result property="name" column="name"/> <result property="email" column="email"/> <association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/> </resultMap
2. association定义
assocation用于MyBatis定义1对1的关联关系
<association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/>
com.mybatis3.mappers.AddressMapper.selectAddressById是一个查询操作标识符，定义在Address-Mapper.xml中
<?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">
<mapper namespace="com.mybatis3.mappers.AddressMapper">
<resultMap type="Address" id="AddressResult">
<id property="addrId" column="addr_id"/>
<result property="street" column="street"/>
<result property="city" column="city"/>
<result property="state" column="state"/>
<result property="zip" column="zip"/>
<result property="country" column="country"/>
</resultMap>
<select id="selectAddressById" parameterType="int" resultMap="AddressResult">
select * from addresses where addr_id=#{addrId}
</select>
</mapper>
3. SQL查询语句
<select id="findStudentById" parameterType="int" resultMap="StudentWithAddressNestedSelect">
select * from STUDENTS where stud_id=#{studId}
</select>
One-To-One映射：使用association内部定义映射（无法重用，不推荐）
1. resultMap定义
<resultMap type="Student" id="StudentWithAddressNestedResultMap"> <id property="studId" column="stud_id"/> <result property="name" column="name"/> <result property="email" column="email"/> <association property="address" javaType="Address"> <id property="addrId" column="addr_id"/> <result property="street" column="street"/> <result property="city" column="city"/> <result property="state" column="state"/> <result property="zip" column="zip"/> <result property="country" column="country"/> </association> </resultMap>
2. assocation内部定义映射关系，javaType的Address是一个类型别名，引用了Address类
<association property="address" javaType="Address"> <id property="addrId" column="addr_id"/> <result property="street" column="street"/> <result property="city" column="city"/> <result property="state" column="state"/> <result property="zip" column="zip"/> <result property="country" column="country"/> </association>
3. SQL语句的写法：
<select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressNestedResultMap">
select stud_id, name, email,phone, a.addr_id, street, city, state, zip, country
FROM STUDENTS s left outer join ADDRESSES a on s.addr_id=a.addr_id
where stud_id=#{studId}
</select>
懒加载配置
要在第二种方式(assocation+select的方式)使用懒加载的方式，需要在MyBatis的主配置文件中配置懒加载选项：
<settings> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings>
总结
1. 实现1对1映射的三种方式
- resultMap继承, join查询，不能懒加载
- assocation+select， 简单查询(N+1)，可以懒加载
- assocation+内嵌的resultMap映射，join查询，不能懒加载
2. 实际工作中，尽量使用assocation+select，加懒加载的方式
3. 工作中，数据库设计的不是那么的严格,比如不设置外键；对于这种关联关系的查询，使用多次的查询，每次执行简单语句，这是Bad Practices