（十六）mybatis多對一關聯查詢
阿新 • • 發佈：2018-11-28
注：程式碼已託管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，專案是mybatis-12-many2one，需要自取，需要配置maven環境以及mysql環境(sql語句在resource下的test.sql中)，覺得有用可以點個小星星，小菜鳥在此Thanks~
一對多關聯查詢：每個國家有很多大臣，領導人，現在我們查詢一個領導，希望能將他所在國家的資訊連帶出來，這就是一對多關聯查詢。
資料表如下:
建立資料庫/表
#建立資料庫
CREATE DATABASE `test` DEFAULT CHARACTER 實體類
country類：
public class Country {
private Integer cid;
private String cname;
public Integer getCid() {
return cid;
}
public void setCid(Integer cid) {
this.cid = cid;
}
public String getCname() {
return cname;
}
public void setCname(String cname) {
this.cname = cname;
}
@Override
public String toString() {
return "Country{" +
"cid=" + cid +
", cname='" + cname + '\'' +
'}';
}
}minister類,在這個類裡面使用country當屬性。
public class Minister {
private Integer mid;
private String mname;
private Country country;
public Integer getMid() {
return mid;
}
public void setMid(Integer mid) {
this.mid = mid;
}
public String getMname() {
return mname;
}
public void setMname(String mname) {
this.mname = mname;
}
public Country getCountry() {
return country;
}
public void setCountry(Country country) {
this.country = country;
}
@Override
public String toString() {
return "Minister{" +
"mid=" + mid +
", mname='" + mname + '\'' +
", country=" + country +
'}';
}
}工具類,獲取sqlsession例項：
public class MyBatisUtils {
/* public SqlSession getSqlSession(){
InputStream is;
try {
is = Resources.getResourceAsStream("mybatis.xml");
return new SqlSessionFactoryBuilder().build(is).openSession();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}*/
static private SqlSessionFactory sqlSessionFactory;
static public SqlSession getSqlSession() {
InputStream is;
try {
is = Resources.getResourceAsStream("mybatis.xml");
if (sqlSessionFactory == null) {
sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
}
return sqlSessionFactory.openSession();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}
}在mybatis.xml主配置檔案裡面註冊mapper檔案：
<mappers>
<mapper resource="mapper/mapper.xml"/>
</mappers>介面的宣告：
public interface IMinisterDao {
Minister selectMinisterById(int mid);
Minister selectMinisterById2(int mid);
}mapper檔案最重要的部分,與一對一有點像，兩種查詢方式，一種是巢狀結果，一種是巢狀查詢，巢狀查詢可以使用懶載入模式，比較常用：
<?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="dao.IMinisterDao">
<!-- resultMap 能解決欄位和屬性不一樣的問題 -->
<!-- 以後用得比較多 ，是因為可以使用延遲載入-->
<!-- 巢狀查詢 -->
<select id="selectCountryById" resultType="Country">
select * from country where cid=#{countryId}
</select>
<resultMap type="Minister" id="countryMapper">
<id column="mid" property="mid"/>
<result column="mname" property="mname"/>
<!-- country中有一個成員變數是ministers，它的泛型是Minister -->
<collection property="country"
ofType="Country"
select="selectCountryById"
column="countryId">
</collection>
</resultMap>
<select id="selectMinisterById" resultMap="countryMapper">
select mid,mname,countryId
from minister
where
mid=#{mid}
</select>
<!-- 巢狀結果-->
<select id="selectMinisterById2" resultMap="countryMapper2">
select mid,mname,countryId,cid,cname
from minister,country
where
mid=#{mid} and countryId = cid
</select>
<resultMap id="countryMapper2" type="Minister">
<id column="mid" property="mid"/>
<result column="mname" property="mname"/>
<collection property="country" ofType="Country">
<id property="cid" column="cid"/>
<result property="cname" column="cname"/>
</collection>
</resultMap>
</mapper>單元測試：
public class MyTest {
private IMinisterDao dao;
private SqlSession sqlSession;
@Before
public void Before(){
sqlSession=MyBatisUtils.getSqlSession();
dao=sqlSession.getMapper(IMinisterDao.class);
}
@Test
public void TestselectMinisterById(){
Minister minister = dao.selectMinisterById(1);
System.out.println(minister);
}
@Test
public void TestselectMinisterById2(){
Minister minister = dao.selectMinisterById2(1);
System.out.println(minister);
}
@After
public void after(){
if(sqlSession!=null){
sqlSession.close();
}
}
}結果：
[service] 2018-07-12 20:07:23,875 - dao.IMinisterDao.selectMinisterById2 -567 [main] DEBUG dao.IMinisterDao.selectMinisterById2 - ==> Preparing: select mid,mname,countryId,cid,cname from minister,country where mid=? and countryId = cid
[service] 2018-07-12 20:07:23,909 - dao.IMinisterDao.selectMinisterById2 -601 [main] DEBUG dao.IMinisterDao.selectMinisterById2 - ==> Parameters: 1(Integer)
[service] 2018-07-12 20:07:23,930 - dao.IMinisterDao.selectMinisterById2 -622 [main] DEBUG dao.IMinisterDao.selectMinisterById2 - <== Total: 1
Minister{mid=1, mname='aaa', country=Country{cid=1, cname='USA'}}