在写xml配置文件中的sql语句时，需要先去数据库工具进行查询，将查询之后的结果来写代码，比较不容易混淆。而且，查询之后的结果的字段名对应resultMap中
一、两边查询：association一对一
association标签用于一对一关系
association的作用是：两张表的关联查询，将另一张表的字段关联过来，然后一起映射到实体类中
例子：作者和博客的关系：一对一（假设每个作者只能有一个博客）
author表和blog表.png
两表一对一查询结果.png
进行两表查询时，我们先在数据库管理工具上进行查询，然后将查询后的表与java代码进行映射
1. 实体类
Author.java
public class Author {
private Integer id;
private String authorName;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getAuthorName() {
return authorName;
}
public void setAuthorName(String authorName) {
this.authorName = authorName;
}
@Override
public String toString() {
return "Author{" +
"id=" + id +
", authorName='" + authorName + '\'' +
'}';
}
}
Blog.java
public class Blog {
private Integer id;
private String blogName;
private Author author;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getBlogName() {
return blogName;
}
public void setBlogName(String blogName) {
this.blogName = blogName;
}
public Author getAuthor() {
return author;
}
public void setAuthor(Author author) {
this.author = author;
}
@Override
public String toString() {
return "Blog{" +
"id=" + id +
", blogName='" + blogName + '\'' +
", author=" + author +
'}';
}
}
2. 配置文件
jdbc.properties
jdbc.driver = com.mysql.jdbc.Driver
jdbc.url = jdbc:mysql://localhost:3306/blog?serverTimezone=UTC
jdbc.username = root
jdbc.password = msj
mybatis配置文件mybatis01.xml
mybatis映射文件mapper01.xml
注意：
3.mapper接口和测试类
BlogMapper.java
public interface BlogMapper {
List findAll();
}
TestDemo.java
public class TestDemo {
@Test
public void test() throws IOException {
InputStream is = Resources.getResourceAsStream("mybatis01.xml");
SqlSessionFactory build = new SqlSessionFactoryBuilder().build(is);
SqlSession session = build.openSession();
BlogMapper mapper = session.getMapper(BlogMapper.class);
List list = mapper.findAll();
for(Blog b:list){
System.out.println(b);
}
}
}
运行结果
Blog{id=1, blogName='aablog', author=Author{id=4, authorName='aa'}}
Blog{id=2, blogName='bbblog', author=Author{id=5, authorName='bb'}}
Blog{id=3, blogName='aablog', author=Author{id=6, authorName='cc'}}
二、两边查询：association多对一
association也可以用在多对一的关系上
例子：文章和博客的关系：多对一
文章和博客表.png
1.实体类
Article.java
//这里省略set,get，toString方法，自己添加
private Integer id;
private String articleName;
private Blog blog;
Blog.java
private Integer id;
private String blogName;
2.配置类（同上）
这里要在mybatis01.xml进行添加配置
Mapper02.xml
3. 测试
ArticleMapper.java
public interface ArticleMapper {
List findAll();
}
TestDemo.java
public class TestDemo {
@Test
public void test() throws IOException {
InputStream is = Resources.getResourceAsStream("mybatis01.xml");
SqlSessionFactory build = new SqlSessionFactoryBuilder().build(is);
SqlSession session = build.openSession();
ArticleMapper mapper = session.getMapper(ArticleMapper.class);
List all = mapper.findAll();
for(Article a:all){
System.out.println(a);
}
}
}
结果查询：
Article{id=1, articleName='111', blog=Blog{id=1, blogName='aablog'}}
Article{id=2, articleName='222', blog=Blog{id=1, blogName='aablog'}}
Article{id=3, articleName='333', blog=Blog{id=1, blogName='aablog'}}
Article{id=4, articleName='444', blog=Blog{id=2, blogName='bbblog'}}
Article{id=5, articleName='555', blog=Blog{id=2, blogName='bbblog'}}
Article{id=6, articleName='666', blog=Blog{id=3, blogName='ccblog'}}
三、两边查询：collection一对多
collection用在一对多的关系上
例子：博客和文章的关系：一对多
1.实体类
Blog.java
注意：这里博客与文章是一对多的关系，所以文章需要用List
private Integer id;
private String blogName;
private List article;
Article.java
private Integer id;
private String articleName;
2.配置类（同上）
这里要在mybatis01.xml进行添加配置
Mapper03.xml
注意：
3. 测试
TestDemo.java
public class TestDemo {
@Test
public void test() throws IOException {
InputStream is = Resources.getResourceAsStream("mybatis01.xml");
SqlSessionFactory build = new SqlSessionFactoryBuilder().build(is);
SqlSession session = build.openSession();
BlogMapper mapper = session.getMapper(BlogMapper.class);
List blogList = mapper.findAll();
for(Blog b:blogList){
System.out.println(b);
}
}
}
测试结果：
Blog{id=1, blogName='aablog', article=[Article{id=1, articleName='111'}, Article{id=2, articleName='222'}, Article{id=3, articleName='333'}]}
Blog{id=2, blogName='bbblog', article=[Article{id=4, articleName='444'}, Article{id=5, articleName='555'}]}
Blog{id=3, blogName='ccblog', article=[Article{id=6, articleName='666'}]}