MyBatis多表查询

1. 一对一查询
1.1一对一查询的模型

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。

1.2 一对一查询的语句

对应的sql语句：select * from orders o,user u where o.uid=u.id;

查询的结果如下：

1.3 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;

}

1.4 创建OrderMapper接口

public interface OrderMapper {
    List<Order> findAll();
}

1.5 配置OrderMapper.xml

<mapper namespace="mapper.OrderMapper">
    <resultMap id="orderMap" type="Order">
    	<!--手动指定字段与实体属性的映射关系
			column:数据表的字段名称
			property:实体的属性名称
		-->
        <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>

其中，resultMap还可以配置如下：

<resultMap id="orderMap" type="Order">
    <result property="id" column="id"></result>
    <result property="ordertime" column="ordertime"></result>
    <result property="total" column="total"></result>
    
    <!--property:当前实体(order)中的属性名称(private User user)
        javaType:当前实体(order)中的属性的类型(User)
    -->
    <association property="user" javaType="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>

1.6 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

2.1 一对多查询的模型

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。
一对多查询的需求：查询一个用户，与此同时查询出该用户具有的订单。

2.2 一对多查询的语句

对应的sql语句：select *,o.id oid from user u left join orders o on u.id=o.uid;
查询的结果如下：

2.3 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<Order> orderList;
}

2.4 创建UserMapper接口

public interface UserMapper {
    List<User> findAll();
}

2.5 配置UserMapper.xml

<mapper namespace="mapper.UserMapper">
    <resultMap id="userMap" type="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>
        
       <!--配置集合信息
			property:集合名称
			ofType:当前集合中的数据类型
		-->
        <collection property="orderList" ofType="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>

2.6 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("----------------------------------");
}

3.多对多查询

3.1 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户。

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。

3.2 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_role ur on u.id=ur.user_id
inner join role r on ur.role_id=r.id;
查询的结果如下：

3.3 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<Order> orderList;
    //代表当前用户具备哪些角色
    private List<Role> roleList;
}

public class Role {
    private int id;
    private String rolename;
}

3.4 添加UserMapper接口方法

List<User> findAllUserAndRole();

3.5 配置UserMapper.xml

<resultMap id="userRoleMap" type="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="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>

3.6 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("----------------------------------");
}

4.知识小结

MyBatis多表配置方式：

一对一配置：使用 resultMap 做配置

一对多配置：使用resultMap + collection 做配置

多对多配置：使用 resultMap + collection 做配置