一、前言
上一章节《老司机学习MyBatis之如何使用association解决一对一关联查询》,我们介绍了MyBatis使用association通过嵌套查询和分段查询两种方式在查询User表的时候关联查询出Role表的信息,我们发现association针对的是一对一的这种关联,那么如果是一对多的关系呢?比如:查询部门信息时,查询出部门下面的所有员工信息呢?这个时候我们使用association是不能实现我们的要求,这时候我们要使用另外一个属性collection来实现这个要求。我们使用collection时候也跟前面使用association时一样,包括嵌套查询和分段查询两种方式,下面我们通过代码案例来说明这两种方式是如何配置的。
二、案例
完整的工程目录如下:
在MySQL数据库新建两张数据库表t_emp和t_dept,并插入若干条数据
CREATE TABLE t_emp ( id int(10) NOT NULL AUTO_INCREMENT, emp_name varchar(100) DEFAULT NULL, dept_id int(10), PRIMARY KEY (id) ) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8; INSERT INTO t_emp(emp_name,dept_id) VALUES ('queen', 1); INSERT INTO t_emp(emp_name,dept_id) VALUES ('king', 1); INSERT INTO t_emp(emp_name,dept_id) VALUES ('tom', 2); INSERT INTO t_emp(emp_name,dept_id) VALUES ('james', 3); INSERT INTO t_emp(emp_name,dept_id) VALUES ('paul', 3); ========================================================= CREATE TABLE t_dept ( id int(10) NOT NULL AUTO_INCREMENT, dept_name varchar(20) DEFAULT NULL, PRIMARY KEY (id) ) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8; INSERT INTO t_dept(dept_name) VALUES ('开发部'); INSERT INTO t_dept(dept_name) VALUES ('市场部'); INSERT INTO t_dept(dept_name) VALUES ('行政部');
Emp实体类
public class Emp { // ID,唯一性 private int id; // 用户名 private String empName; // 部门ID private int deptId; public Emp() { } public Emp(int id, String empName, int deptId) { this.id = id; this.empName = empName; this.deptId = deptId; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getEmpName() { return empName; } public void setEmpName(String empName) { this.empName = empName; } public int getDeptId() { return deptId; } public void setDeptId(int deptId) { this.deptId = deptId; } @Override public String toString() { return "Emp [id=" + id + ", empName=" + empName + ", deptId=" + deptId + "]"; } }
Dept实体类
public class Dept { private int id; private String deptName; private List<Emp> empList; public Dept() { } public Dept(int id, String deptName, List<Emp> empList) { this.id = id; this.deptName = deptName; this.empList = empList; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getDeptName() { return deptName; } public void setDeptName(String deptName) { this.deptName = deptName; } public List<Emp> getEmpList() { return empList; } public void setEmpList(List<Emp> empList) { this.empList = empList; } @Override public String toString() { return "Dept [id=" + id + ", deptName=" + deptName + "]"; } }配置文件log4j.properties
log4j.rootLogger=DEBUG,A1 log4j.logger.com.queen = DEBUG log4j.logger.org.mybatis = DEBUG log4j.appender.A1=org.apache.log4j.ConsoleAppender log4j.appender.A1.layout=org.apache.log4j.PatternLayout log4j.appender.A1.layout.ConversionPattern=%-d{yyyy-MM-dd HH:mm:ss,SSS} [%t] [%c]-[%p] %m%n
配置文件db.properties
jdbc.driver=com.mysql.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/mybatis jdbc.username=root jdbc.password=root配置文件mybatis-config.xml
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <properties resource="db.properties"></properties> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/mybatis" /> <property name="username" value="root" /> <property name="password" value="root" /> </dataSource> </environment> </environments> <mappers> <mapper resource="DeptMapper.xml" /> </mappers> </configuration>
新建接口类DeptMapper.java,增加接口方法getDeptById
public interface DeptMapper { /** * 根据ID查询Dept信息,返回一条记录Dept * @param id * @return */ public Dept getDeptById(int id); }
首先我们来看一下嵌套查询,DeptMapper.xml配置文件如下
<?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.queen.mybatis.mapper.DeptMapper"> <resultMap type="com.queen.mybatis.bean.Dept" id="deptMap"> <id column="mid" property="id"/> <result column="deptName" property="deptName"/> <!-- collection定义集合类型的属性封装规则 ofType:指定集合里面的元素类型 --> <collection property="empList" ofType="com.queen.mybatis.bean.Emp"> <!-- 定义这个集合中元素的封装规则 --> <id column="nid" property="id"/> <result column="empName" property="empName"/> <result column="deptId" property="deptId"/> </collection> </resultMap> <select id="getDeptById" resultMap="deptMap"> SELECT m.id mid, m.dept_name deptName,n.id nid, n.emp_name empName,n.dept_id deptId FROM t_dept m LEFT JOIN t_emp n ON m.id = n.dept_id WHERE m.id = #{id} </select> </mapper>
新增测试类MyBatisTest,添加测试方法testGetDeptByCollection
@Test public void testGetDeptByCollection() throws IOException { SqlSessionFactory sqlSessionFactory = getSqlSessionFactory(); SqlSession openSession = sqlSessionFactory.openSession(); try { DeptMapper mapper = openSession.getMapper(DeptMapper.class); Dept dept = mapper.getDeptById(1); System.out.println(dept); System.out.println(dept.getEmpList()); } finally { openSession.close(); } }
控制台打印结果如下
2017-08-10 22:20:32,961 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptById]-[DEBUG] ==> Preparing: SELECT m.id mid, m.dept_name deptName,n.id nid, n.emp_name empName,n.dept_id deptId FROM t_dept m LEFT JOIN t_emp n ON m.id = n.dept_id WHERE m.id = ? 2017-08-10 22:20:33,019 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptById]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-10 22:20:33,070 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptById]-[DEBUG] <== Total: 2 Dept [id=1, deptName=开发部] [Emp [id=1, empName=queen, deptId=1], Emp [id=2, empName=king, deptId=1]]
这样我们可以通过collection实现一对多这种情况的数据封装
接下来我们介绍第二种:使用collection进行分步查询,跟上个章节使用assocication一样,即我们查完Dept表数据后,我们在根据查到的Dept的ID查询Emp表数据。分段分多条SQL执行。
新建一个接口类EmpMapper,新增接口方法getEmpListByDeptId
public interface EmpMapper { /** * 根据部门ID查询该部门下的所有员工信息 * @param id * @return */ public List<Emp> getEmpListByDeptId(int deptId); }
新建一个EmpMapper.xml文件,配置如下
<?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.queen.mybatis.mapper.EmpMapper"> <select id="getEmpListByDeptId" resultType="com.queen.mybatis.bean.Emp"> select id,emp_name empName,dept_id deptId from t_emp where dept_id=#{deptId} </select> </mapper>
修改DeptMapper文件,新增一个方法getDeptByIdStep
/** * 根据ID查询Dept信息,返回一条记录Dept,分步查询 * @param id * @return */ public Dept getDeptByIdStep(int id);
修改DeptMapper.xml文件,增加如下配置
<!-- 以下使用collection的分步查询 --> <resultMap type="com.queen.mybatis.bean.Dept" id="deptMapStep"> <id column="id" property="id"/> <result column="dept_name" property="deptName"/> <!--将查到的部门ID传递给getEmpListByDeptId方法--> <collection property="empList" select="com.queen.mybatis.mapper.EmpMapper.getEmpListByDeptId" column="id"> </collection> </resultMap> <select id="getDeptByIdStep" resultMap="deptMapStep"> select id, dept_name from t_dept where id=#{id} </select>
修改测试类MyBatisTest,添加测试方法testGetDeptByCollectionStep
@Test public void testGetDeptByCollectionStep() throws IOException { SqlSessionFactory sqlSessionFactory = getSqlSessionFactory(); SqlSession openSession = sqlSessionFactory.openSession(); try { DeptMapper mapper = openSession.getMapper(DeptMapper.class); Dept deptStep = mapper.getDeptByIdStep(1); System.out.println(deptStep); System.out.println(deptStep.getEmpList()); } finally { openSession.close(); } }
注意:DeptMapper.xml和EmpMapper.xml等映射文件都用配置到mybatis-config.xml文件中,否则会报错。
<mappers> <mapper resource="DeptMapper.xml" /> <mapper resource="EmpMapper.xml" /> </mappers>
控制台打印结果如下
2017-08-10 22:30:15,158 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptByIdStep]-[DEBUG] ==> Preparing: select id, dept_name from t_dept where id=? 2017-08-10 22:30:15,217 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptByIdStep]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-10 22:30:15,280 [main] [com.queen.mybatis.mapper.EmpMapper.getEmpListByDeptId]-[DEBUG] ====> Preparing: select id,emp_name empName,dept_id deptId from t_emp where dept_id=? 2017-08-10 22:30:15,281 [main] [com.queen.mybatis.mapper.EmpMapper.getEmpListByDeptId]-[DEBUG] ====> Parameters: 1(Integer) 2017-08-10 22:30:15,283 [main] [com.queen.mybatis.mapper.EmpMapper.getEmpListByDeptId]-[DEBUG] <==== Total: 2 2017-08-10 22:30:15,284 [main] [com.queen.mybatis.mapper.DeptMapper.getDeptByIdStep]-[DEBUG] <== Total: 1 Dept [id=1, deptName=开发部] [Emp [id=1, empName=queen, deptId=1], Emp [id=2, empName=king, deptId=1]]从上述打印结果,可以看到控制台打印了两段SQL语句,一句用来查询Dept部门信息,另外一句用来查询Emp员工信息,实现了分步查询的效果,且控制台打印结果正常。
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