mybatis入门之四：实现关联数据的查询

有了前面几章的基础，对一些简单的应用是可以处理的，但在实际项目中，经常是关联表的查询，比如最常见到的多对一，一对多等。这些查询是如何处理的呢，这一讲就讲这个问题。我们首先创建一个角色这个表，并初始化数据.

CREATE TABLE `roleinfo` (
  `roleId` int(11) NOT NULL AUTO_INCREMENT,
  `roleName` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`roleId`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8


-- 添加几条测试数据

INSERT into roleinfo(roleId, roleName) values(1, "admin");
INSERT into roleinfo(roleId, roleName) values(2, "user");
INSERT into roleinfo(roleId, roleName) values(3, "customer");
INSERT into roleinfo(roleId, roleName) values(4, "other");

按照一般需求，一个用户可以拥有多个角色，因此产生第三张表userandrole

CREATE TABLE `userandrole` (
  `userAndRoleId` int(11) NOT NULL AUTO_INCREMENT,
  `adminId` int(11) DEFAULT NULL,
  `roleId` int(11) DEFAULT NULL,
  PRIMARY KEY (`userAndRoleId`),
  KEY `FK_Reference_25` (`adminId`),
  KEY `FK_Reference_26` (`roleId`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8


-- 添加几条测试数据

INSERT into userandrole(userAndRoleId, adminId, roleId) values(1, 1, 1);
INSERT into userandrole(userAndRoleId, adminId, roleId) values(2, 1, 2);
INSERT into userandrole(userAndRoleId, adminId, roleId) values(3, 1, 3);

对应实体类

package com.handly.mybatis.model;

/**
 * 角色信息
 * @author handly
 */
public class RoleInfo {
	private Integer roleId;
	private String roleName;

	//省略get、set方法

}

package com.handly.mybatis.model;

import java.util.List;

public class UserAndRole {
	private Integer userAndRoleId;
	private Integer adminId;
	private Integer roleId;
	
	private User user;
	private List<RoleInfo> roles;
	
	public UserAndRole(){}
	
	@Override
	public String toString() {
		return "Classes [id=" + userAndRoleId + ", user=" + user + ", roles=" + roles + "]";
	}

	//省略get/set方法

}

方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集封装联表查询的数据(去除重复的数据)

<!-- 方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据) 
		SELECT * from user u, roleinfo r, userandrole ur where ur.adminId=u.id and ur.roleId=r.roleId and u.id=1 -->
	<select id="getRole1" parameterType="int" resultMap="UserRoleResultMap">
		SELECT * from user u, roleinfo r, userandrole ur where ur.adminId=u.id and ur.roleId=r.roleId and u.id=#{id}
	</select>
	<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
	<resultMap type="com.handly.mybatis.model.UserAndRole" id="UserRoleResultMap">
		<id property="userAndRoleId" column="userAndRoleId" />
		<result property="adminId" column="adminId" />
		<result property="roleId" column="roleId" />
		<association property="user" javaType="com.handly.mybatis.model.User">
			<id property="id" column="id" />
			<result property="username" column="username" />
			<result property="password" column="password" />
			<result property="name" column="name" />
		</association>
		<!-- ofType指定students集合中的对象类型 -->
		<collection property="roles" ofType="com.handly.mybatis.model.RoleInfo">
			<id property="roleId" column="roleId" />
			<result property="roleName" column="roleName" />
		</collection>
	</resultMap>

方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型

<!-- 方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型 SELECT * from userandrole ur where ur.adminId=1; 
		SELECT id, username from user where id=1 //1 是上一个查询得到的adminId的值
		SELECT roleId, roleName from roleinfo where roleId=2 //2是第一个查询得到的roleId字段的值 -->
	<select id="getRole2" parameterType="int" resultMap="UserRoleResultMap2">
		SELECT * from userandrole ur where ur.adminId=#{id}
	</select>
	<resultMap type="com.handly.mybatis.model.UserAndRole" id="UserRoleResultMap2">
		<id property="userAndRoleId" column="userAndRoleId" />
		<result property="adminId" column="adminId" />
		<result property="roleId" column="roleId" />
		<association property="user" column="adminId" javaType="com.handly.mybatis.model.User" select="getUser"></association>
		<collection property="roles" column="roleId" ofType="com.handly.mybatis.model.RoleInfo" select="getRoles"></collection>
	</resultMap>
	<select id="getUser" parameterType="int" resultType="com.handly.mybatis.model.User">
		SELECT id, username from user where id=#{id}
	</select>
	<select id="getRoles" parameterType="int" resultType="com.handly.mybatis.model.RoleInfo">
		SELECT roleId, roleName from roleinfo where roleId=#{id}
	</select>

新加接口

package com.handly.mybatis.dao;

import java.util.List;
import com.handly.mybatis.model.UserAndRole;

public interface RoleInfoDao {
	public List<UserAndRole> getRole1(Integer id);
	
	public List<UserAndRole> getRole2(Integer id);
}

测试代码

/**
	 * 获取用户角色列表 
	 */
	@Test
	public void testGetRoleListByUserId(){
		SqlSession session = sqlSessionFactory.openSession();
        try {
            RoleInfoDao roleDao = session.getMapper(RoleInfoDao.class);
//            List<UserAndRole> list = roleDao.getRole1(1);
            List<UserAndRole> list = roleDao.getRole2(1);
            if(null!=list && list.size()>0){
            	for(UserAndRole ur : list){
//            		System.out.println(ur.getDomainId());
            		System.out.println("u:" + ur.getUser().getUsername());
            		if(null!=ur.getRoles() && ur.getRoles().size()>0){
            			for(RoleInfo r : ur.getRoles()){
            				System.out.print(r.getRoleName() + ", ");
            			}
            		}
            	}
            }
        } finally {
            session.close();
        }
	}

mybatis入门之四：实现关联数据的查询

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