一对一查询可以用注解`@OneToOne`来完成，默认为`left join`语句。 ```java public class Person implements Serializable { /** * person：对应 IDCard#person 变量名称. */ @OneToOne(mappedBy = "person", fetch = FetchType.EAGER) private IDCard idCard; } ``` ```java public class IDCard implements Serializable { /** * referencedColumnName：对应 Person#id 主键变量名称. * 将会在表 IDCard 建立物理外键 person_id. */ @OneToOne(targetEntity = Person.class) @JoinColumn(name = "person_id", referencedColumnName = "id") private Person person; } ```