Processing complex business logic of MyBatis multi-table query: example demonstration

MyBatis multi-table query example: How to handle complex business logic of related table data, specific code examples are needed

In development, we often face the need to query multiple The situation of related tables. In this case, in order to meet complex business requirements, we need to process some business logic of related table data while querying. This article will introduce how to use MyBatis to perform multi-table queries and give specific code examples.

First, we need to create relevant data tables and entity classes. Suppose we have two tables: one is the user table and the other is the order table. There is a relationship between these two tables, and a user can have multiple orders.

First we create the user table user:

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
)

Then we create the order table order:

CREATE TABLE `order` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `amount` decimal(10,2) NOT NULL,
  PRIMARY KEY (`id`),
  INDEX `fk_user_id` (`user_id`),
  CONSTRAINT `fk_user_id` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`)
)

Next, we create the corresponding entity classes User and Order:

User.java：

public class User {
    private int id;
    private String name;
    // 省略getter和setter方法
}

Order.java：

public class Order {
    private int id;
    private User user;
    private BigDecimal amount;
    // 省略getter和setter方法
}

Next, we need to write the query Mapper interface and the corresponding Mapper XML file. Suppose we need to query users and their order list, this can be achieved in the following way.

UserMapper.java：

public interface UserMapper {
    User getUserById(int id);

    List<Order> getOrdersByUserId(int userId);
}

UserMapper.xml：

<!-- 查询用户 -->
<select id="getUserById" parameterType="int" resultType="User">
    SELECT * FROM `user` WHERE id = #{id}
</select>

<!-- 查询订单 -->
<select id="getOrdersByUserId" parameterType="int" resultMap="OrderMap">
    SELECT o.id as order_id, o.amount, u.id as user_id, u.name
    FROM `order` o
    INNER JOIN `user` u ON o.user_id = u.id
    WHERE o.user_id = #{userId}
</select>

<!-- 定义ResultMap -->
<resultMap id="OrderMap" type="Order">
    <id property="id" column="order_id"/>
    <result property="amount" column="amount"/>
    <association property="user" column="user_id" javaType="User"
                 resultMap="UserResultMap"/>
</resultMap>

<!-- 定义User的ResultMap -->
<resultMap id="UserResultMap" type="User">
    <id property="id" column="user_id"/>
    <result property="name" column="name"/>
</resultMap>

In the above code, we associate two tables through INNER JOIN and use ResultMap to query The result is mapped to the Order object. At the same time, we also define a User ResultMap to map query results to User objects.

Now we can use MyBatis to perform multi-table queries and process complex business logic of related table data.

SqlSessionFactory sqlSessionFactory = // 获取SqlSessionFactory
SqlSession sqlSession = sqlSessionFactory.openSession();
try {
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
    
    User user = userMapper.getUserById(1);
    List<Order> orders = userMapper.getOrdersByUserId(1);
    
    user.setOrders(orders); // 将订单列表设置到用户对象中
    
    // 处理业务逻辑
    for (Order order : orders) {
        // ...
    }
} finally {
    sqlSession.close();
}

In the above code, we first query the user information of the specified id through the getUserById method, and then query the user's order list through the getOrdersByUserId method. Finally, we set the order list into the user object in order to handle complex business logic.

Through the above sample code, we can see that the complex business logic of using MyBatis to perform multi-table queries and process related table data is not complicated. By properly designing the Mapper interface and Mapper XML file, we can easily implement such a function.

Summary, this article introduces how to use MyBatis to perform multi-table queries and process complex business logic of related table data, and gives specific code examples. I hope this will be helpful to you when dealing with multi-table queries in actual development.

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