1.Mybatis多表查询

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,user u 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;
    //当前订单属于哪一个用户
    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 {
    //查询全部的方法
    public List<Order> findAll();
}

5. 配置OrderMapper.xml

<mapper namespace="com.itheima.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
        -->
        <id column="oid" property="id"></id>
        <result column="ordertime" property="ordertime"></result>
        <result column="total" property="total"></result>
        <!--<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>-->
        <!--
            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">
            <id column="uid" property="id"></id>
            <result column="username" property="username"></result>
            <result column="password" property="password"></result>
            <result column="birthday" property="birthday"></result>
        </association>
    </resultMap>
    <select id="findAll" resultMap="orderMap">
        SELECT *,o.id oid 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> orderList = mapper.findAll();
for (Order order : orderList) {
    System.out.println(order);
}

1.2 一对多查询

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. 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：

3. 修改User实体

public class Order {
    private int id;
    private Date ordertime;
    private double total;
    //当前订单属于哪一个用户
    private User user;

public class User {
    private int id;
    private String username;
    private String password;
    private Date birthday;
    //描述的是当前用户存在哪些订单
    private List<Order> orderList;

4. 创建UserMapper接口

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

5. 配置UserMapper.xml

<mapper namespace="com.itheima.mapper.UserMapper">
    <resultMap id="userMap" type="user">
        <id column="uid" property="id"></id>
        <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">
            <!--封装order的数据-->
            <id column="oid" property="id"></id>
            <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,orders o WHERE u.id=o.uid
    </select>
</mapper>

6. 测试结果

UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List<User> orderList = mapper.findAll();
for (User user : orderList) {
    System.out.println(user);
}

输出

User{id=1, username='zhangsan', password='123', birthday=null, orderList=[Order{id=1, ordertime=Tue Apr 12 16:52:52 CST 2022, total=3000.0, user=null}, Order{id=2, ordertime=Sun Jun 12 16:53:12 CST 2022, total=5800.0, user=null}]}
User{id=2, username='wangwu', password='123', birthday=Tue Apr 12 17:33:43 CST 2022, orderList=[Order{id=3, ordertime=Fri Apr 29 16:53:37 CST 2022, total=2456.0, user=null}]}
User{id=6, username='ceshi', password='abc', birthday=Tue Apr 12 17:33:37 CST 2022, orderList=[Order{id=4, ordertime=Tue Feb 12 17:21:51 CST 2019, total=9999.0, user=null}]}

1.3 多对多查询

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 * FROM USER u,sys_user_role ur,sys_role r WHERE u.id=ur.userId AND ur.roleId=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<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;
}

4. 添加UserMapper接口方法

List<User> findAllUserAndRole();

5. 配置UserMapper.xml

<resultMap id="userRoleMap" type="user">
    <!--user的信息-->
    <id column="userId" property="id"></id>
    <result column="username" property="username"></result>
    <result column="password" property="password"></result>
    <result column="birthday" property="birthday"></result>
    <!--user内部的roleList信息-->
    <collection property="roleList" ofType="role">
        <id column="roleId" property="id"></id>
        <result column="roleName" property="roleName"></result>
        <result column="roleDesc" property="roleDesc"></result>
    </collection>
</resultMap>
<select id="findUserAndRoleAll" resultMap="userRoleMap">
    SELECT * FROM USER u,sys_user_role ur,sys_role r WHERE u.id=ur.userId AND ur.roleId=r.id
</select>

6. 测试结果

UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List<User> userAndRoleAll = mapper.findUserAndRoleAll();
for (User user : userAndRoleAll) {
    System.out.println(user);
}

1.4 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