4.1 一对一
4.1.1 表对应关系，
4.1.2 实体对应关系
public class Order {
private int id; private Date ordertime; private double total; //代表当前订单属于哪个用户 private User user; } public class User { private int id; private String username; private String password; private Date birthday; } 4.1.3 配置 OrderMapper.xml文件
<mapper namespace="com.lagou.mapper.OrderMapper">
<resultMap id="orderMap" type="com.lagou.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> #或者还可以这样配置 <resultMap id="orderMap" type="com.lagou.domain.Order"> <result property="id" column="id"></result> <result property="ordertime" column="ordertime"></result> <result property="total" column="total"></result> <association property="user" javaType="com.lagou.domain.User"> <result column="uid" property="id"></result> <result column="username" property="username"></result> <result column="password" property="password"></result> <result column="birthday" property="birthday"></result> </association> </resultMap> 4.2 一对多
4.2.1 表对应关系
查询语句可以写为
select *,o.id oid from user u left join orders o on u.id=o.uid;
4.2.2 实体类关系
public class Order {
private int id; private Date ordertime; private double total; //当前订单属于哪个用户 private User user; } public class User { private int id; private String username; private String password; private Date birthday; //当前用户的订单列表 private List<Order> orderList; } 4.2.3 配置UserMapper.xml
<mapper namespace="com.lagou.mapper.UserMapper">
<resultMap id="userMap" type="com.lagou.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.lagou.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> 4.3 多对多
4.3.1 表对应关系
对应SQL语句
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;
4.3.2 实体类对应关系
public class User {
private int id; private String username; private String password; private Date birthday; //当前用户具备哪些角色 private List<Role> roleList; } public class Role { private int id; private String rolename; } 4.3.3 配置UserMapper.xml文件
<resultMap id="userRoleMap" type="com.lagou.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.lagou.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>
原文：https://www.cnblogs.com/benjaming0321/p/12546839.html