1.1 一对一查询
1. 一对一查询的模型
用户表和订单表的关系为，一个用户有多个订单，一个订单只从属于一个用户
一对一查询的需求：查询一个订单，与此同时查询出该订单所属的用户
2. 一对一查询的语句
对应的sql语句：select * from orders o,useru where o.uid=u.id;
查询的结果如下：
3. 创建Order和User实体
public class Order { private int id; private Date ordertime; private double total; // 代表当前订单从属于哪一个客户 private User user; }
public class User { private int id; private String username; private String password; private Date birthday; }
4. 创建OrderMapper接口
public interface OrderMapper { List<Order> findAll(); }
5. 配置OrderMapper.xml
<m<mapper namespace="com.itheima.mapper.OrderMapper"> <resultMap id="orderMap" type="com.itheima.domain.Order"> <result column="uid" property="user.id"></result> <result column="username" property="user.username"></result> <result column="password" property="user.password"></result> <result column="birthday" property="user.birthday"></result> </resultMap> <select id="findAll" resultMap="orderMap"> select * from orders o,user u where o.uid=u.id </select> </mapper>
6. 测试结果
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); List<Order> all = mapper.findAll(); for(Order order : all){ System. out .println(order); }
2. 一对多查询的语句
对应的sql语句：select *,o.id oidfrom user u left join orders o on u.id=o.uid;
查询的结果如下：
3. 修改User实体
public class Order {
private int id; private Date ordertime; private double total;
// 代表当前订单从属于哪一个客户 private User user;
public class User {
private int id; private String username; private String password; private Date birthday;
// 代表当前用户具备哪些订单
private List orderList;
}
4. 创建UserMapper接口
public interface UserMapper {
List findAll();
}
5. 配置UserMapper.xml
<mapper namespace="com.itheima.mapper.UserMapper"> <resultMap id="userMap" type="com.itheima.domain.User"> <result column="id" property="id"></result> <result column="username" property="username"></result> <result column="password" property="password"></result> <result column="birthday" property="birthday"></result> <collection property="orderList" ofType="com.itheima.domain.Order"> <result column="oid" property="id"></result> <result column="ordertime" property="ordertime"></result> <result column="total" property="total"></result> </collection> </resultMap> <select id="findAll" resultMap="userMap"> select * ,o.id oid from user u left join orders o on u.id=o.uid </select> </mapper>
6. 测试结果
UserMapper mapper = sqlSession.getMapper(UserMapper.class); List<User> all = mapper.findAll(); for(User user : all){ System. out .println(user.getUsername()); List<Order> orderList = user.getOrderList(); for(Order order : orderList){ System. out .println(order); } System. out .println("----------------------------------"); }
6. 测试结果
1. 多对多查询的模型
用户表和角色表的关系为，一个用户有多个角色，一个角色被多个用户使用
多对多查询的需求：查询用户同时查询出该用户的所有角色
2. 多对多查询的语句
对应的sql语句：select u.,r.,r.id rid from user u left join user_roleuron u.id=ur.user_id
inner join role r on ur.role_id=r.id;
查询的结果如下：
3. 创建Role实体，修改User实体
public class User { private int id; private String username; private String password; private Date birthday;
// 代表当前用户具备哪些订单
private List orderList;
// 代表当前用户具备哪些角色
private List roleList;
}
public class Role { private int id; private String rolename;}
4. 添加UserMapper接口方法
List findAllUserAndRole();
5. 配置UserMapper.xml
<resultMap id="userRoleMap" type="com.itheima.domain.User"> <result column="id" property="id"></result> <result column="username" property="username"></result> <result column="password" property="password"></result> <result column="birthday" property="birthday"></result> <collection property="roleList" ofType="com.itheima.domain.Role"> <result column="rid" property="id"></result> <result column="rolename" property="rolename"></result> </collection> </resultMap> <select id="findAllUserAndRole" resultMap="userRoleMap"> select u. * ,r. * ,r.id rid from user u left join user_role ur on u.id=ur.user_id inner join role r on ur.role_id=r.id </select>
6. 测试结果
UserMapper mapper = sqlSession.getMapper(UserMapper.class); List<User> all = mapper.findAllUserAndRole(); for(User user : all){ System. out .println(user.getUsername()); List<Role> roleList = user.getRoleList(); for(Role role : roleList){ System. out .println(role); } System. out .println("----------------------------------"); }
