注:代碼已託管在GitHub
上,地址是:https://github.com/Damaer/Mybatis-Learning
,項目是mybatis-11-one2one
,須要自取,須要配置maven
環境以及mysql
環境(sql
語句在resource
下的test.sql
中),以爲有用能夠點個小星星。java
docsify
文檔地址在:https://damaer.github.io/Mybatis-Learning/#/
mysql
所謂一對一多表查詢,舉個例子:咱們有不少國家,每個國家只有一個領導人(假設),咱們須要根據id
查詢國家信息,帶上領導人的信息。git
設計表的時候,咱們須要考慮因爲是一對多關係,咱們須要在國家表裏面使用一個字段對應領導人的信息。github
#建立數據庫 CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci; #建立數據表 CREATE TABLE `test`.`country` ( `cid` INT(10) NOT NULL AUTO_INCREMENT ,`cname` VARCHAR(20) NOT NULL ,`mid` INT(10) NOT NULL,PRIMARY KEY(`cid`)) ENGINE = MyISAM; CREATE TABLE `test`.`minister` ( `mid` INT(10) NOT NULL AUTO_INCREMENT ,`mname` VARCHAR(20) NOT NULL,PRIMARY KEY(`mid`)) ENGINE = MyISAM; #初始化數據表 INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('1', 'aaa', '1'); INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('2', 'bbb', '2'); INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('3', 'ccc', '3'); INSERT INTO `minister` (`mid`, `mname`) VALUES ('1', 'sam'); INSERT INTO `minister` (`mid`, `mname`) VALUES ('2', 'jane'); INSERT INTO `minister` (`mid`, `mname`) VALUES ('3', 'jone');
數據表以下:sql
country
對應的實體類:shell
public class Country { private Integer cid; private String cname; private Minister minister; 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; } public Minister getMinister() { return minister; } public void setMinister(Minister minister) { this.minister = minister; } @Override public String toString() { return "Country{" + "cid=" + cid + ", cname='" + cname + '\'' + ", minister=" + minister + '}'; } }
Minister
對應的實體類:數據庫
public class Minister { private Integer mid; private String mname; @Override public String toString() { return "Minister [mid=" + mid + ", mname=" + mname + "]"; } 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; } }
MybatisUtils
工具類,用於獲取單例的sqlSession
對象:mybatis
public class MyBatisUtils { 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
裏面註冊mapper
文件:app
<mappers> <mapper resource="mapper/mapper.xml"/> </mappers>
咱們的兩個接口(對應mapper
文件中的兩個sql
語句):maven
public interface ICountryDao { Country selectCountryById(int cid); Country selectCountryById2(int cid); }
mapper
文件,有兩種寫法,一種是嵌套結果,一種是嵌套查詢。
嵌套結果的就是使用一條sql完成查詢,須要在本身定義的resultMap裏面使用<association></association>
來組織結果屬性。
嵌套查詢也有一個<association></association>
,可是在最外面的<select>
標籤並無將全部須要的東西查詢出來,而是在<association></association>
裏面指定須要關聯查詢的sql
語句的id
,這樣的方式就是懶加載:
<?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.ICountryDao"> <!-- resultMap 能解決字段和屬性不同的問題 --> <!-- 之後用得比較多 ,是由於可使用延遲加載--> <!-- 嵌套查詢 --> <select id="selectMinisterByCountry" resultType="Minister"> select mid,mname from minister where mid=#{ooo} </select> <resultMap type="Country" id="countryMapper"> <id column="cid" property="cid"/> <result column="cname" property="cname"/> <!-- country中有一個成員變量是ministers,它的泛型是Minister --> <association property="minister" select="selectMinisterByCountry" column="mid"> </association> </resultMap> <select id="selectCountryById" resultMap="countryMapper"> select cid,cname,mid from country where cid=#{cid} </select> <!-- 嵌套結果--> <select id="selectCountryById2" resultMap="countryMapper2"> select * from country c,minister m where c.mid = m.mid and c.cid= #{cid} </select> <resultMap id="countryMapper2" type="Country"> <id column="cid" property="cid"/> <result column="cname" property="cname"/> <association property="minister" javaType="Minister"> <id property="mid" column="mid"/> <result property="mname" column="mname"/> </association> </resultMap> </mapper>
單元測試:
public class MyTest { private ICountryDao dao; private SqlSession sqlSession; @Before public void Before(){ sqlSession=MyBatisUtils.getSqlSession(); dao=sqlSession.getMapper(ICountryDao.class); } @Test public void TestselectCountryById(){ Country country=dao.selectCountryById(1); System.out.println(country); } @Test public void TestselectCountryById2(){ Country country=dao.selectCountryById2(1); System.out.println(country); } @After public void after(){ if(sqlSession!=null){ sqlSession.close(); } } }
結果:
[service] 2018-07-12 15:23:38,971 - dao.ICountryDao.selectCountryById -537 [main] DEBUG dao.ICountryDao.selectCountryById - ==> Preparing: select cid,cname,mid from country where cid=? [service] 2018-07-12 15:23:39,004 - dao.ICountryDao.selectCountryById -570 [main] DEBUG dao.ICountryDao.selectCountryById - ==> Parameters: 1(Integer) [service] 2018-07-12 15:23:39,037 - dao.ICountryDao.selectMinisterByCountry -603 [main] DEBUG dao.ICountryDao.selectMinisterByCountry - ====> Preparing: select mid,mname from minister where mid=? [service] 2018-07-12 15:23:39,037 - dao.ICountryDao.selectMinisterByCountry -603 [main] DEBUG dao.ICountryDao.selectMinisterByCountry - ====> Parameters: 1(Integer) [service] 2018-07-12 15:23:39,040 - dao.ICountryDao.selectMinisterByCountry -606 [main] DEBUG dao.ICountryDao.selectMinisterByCountry - <==== Total: 1 [service] 2018-07-12 15:23:39,041 - dao.ICountryDao.selectCountryById -607 [main] DEBUG dao.ICountryDao.selectCountryById - <== Total: 1 Country{cid=1, cname='aaa', minister=Minister [mid=1, mname=sam]}
【做者簡介】:
秦懷,公衆號【秦懷雜貨店】做者,技術之路不在一時,山高水長,縱使緩慢,馳而不息。這個世界但願一切都很快,更快,可是我但願本身能走好每一步,寫好每一篇文章,期待和大家一塊兒交流。