mybatis collections

Mybatis中的`collection`标签用于实现一对多的多表查询。它可以在`resultMap`标签内使用，用于关联查询一个Bean中的list属性。例如，在查询用户信息时，如果需要关联查询出角色集合，就可以使用`collection`标签来实现。

`collection`标签有两种使用方法：
1. 方法一：嵌套结果映射。这种方法通过在`collection`标签内使用`resultMap`标签来指定关联查询的结果映射规则。
2. 方法二：嵌套select查询。这种方法通过在`collection`标签内使用`select`语句来进行关联查询。

此外，Mybatis还提供了`association`标签用于一对一的关联查询。你可以在`resultMap`标签内使用`association`标签来指定一对一的关联查询规则。 如果你想了解更多关于`association`的信息，可以参考这篇文章。

综上所述，Mybatis中的`collection`标签可以用于实现一对多的多表查询，而`association`标签则用于实现一对一的关联查询。<span class="em">1</span><span class="em">2</span><span class="em">3</span><span class="em">4</span>

相关问题

mybatis collection

MyBatis provides a way to map relationships between objects in Java and tables in a database using collections. A collection is a group of objects that are related to each other in some way. In MyBatis, collections are used to map one-to-many relationships between objects and tables.

There are three types of collections in MyBatis:

1. List: A list is an ordered collection of objects. In MyBatis, a list is used to map a one-to-many relationship between an object and a table.

2. Set: A set is an unordered collection of objects. In MyBatis, a set is used to map a one-to-many relationship between an object and a table, but the order of the objects is not important.

3. Map: A map is a collection of key-value pairs. In MyBatis, a map is used to map a many-to-one relationship between objects and tables.

To map a collection in MyBatis, you can use the collection element in your mapping file. The collection element contains a property attribute that specifies the name of the collection property in your Java object, and a select attribute that specifies the SQL statement to retrieve the related objects from the database.

For example, if you have a User object that has a list of Orders, you can map the Orders collection in MyBatis like this:

<resultMap id="userResultMap" type="User">
  <id property="id" column="user_id" />
  <result property="username" column="username" />
  <collection property="orders" ofType="Order" select="selectOrdersByUserId"/>
</resultMap>

<select id="selectOrdersByUserId" resultType="Order">
  SELECT * FROM orders WHERE user_id = #{id}
</select>

In this example, the collection element maps the Orders property in the User object to the selectOrdersByUserId SQL statement. MyBatis will execute the SQL statement and populate the Orders list in the User object with the results.

Collections are a powerful feature in MyBatis that allow you to map complex relationships between objects and tables in your database. By using collections, you can simplify your code and make it easier to work with your data in Java.

mybatis 批量更新

MyBatis 支持批量更新操作，可以通过 `update` 标签中的 `foreach` 元素来实现。

假设有一个 `User` 实体类，包含 `id` 和 `name` 属性，我们要批量更新多个用户的姓名，可以按照以下步骤操作：

1. 在 Mapper.xml 文件中添加一个 `update` 标签，设置 `id` 和 `parameterType` 属性：

<update id="batchUpdateUser">
    <foreach collection="users" item="user" separator=";">
        update user set name = #{user.name} where id = #{user.id}
    </foreach>
</update>

2. 在 Java 代码中，调用 `SqlSession` 的 `update` 方法，传入 Mapper.xml 中定义的 `id` 和一个包含多个 `User` 对象的集合：

List<User> userList = new ArrayList<>();
User user1 = new User(1, "Jack");
User user2 = new User(2, "Rose");
userList.add(user1);
userList.add(user2);

try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
    int rows = sqlSession.update("batchUpdateUser", Collections.singletonMap("users", userList));
    sqlSession.commit();
}

在 Mapper.xml 文件中，`foreach` 元素的 `collection` 属性指定了传入的集合变量名，`item` 属性指定了在循环中使用的变量名，`separator` 属性指定了每个 SQL 语句之间的分隔符。

在 Java 代码中，`Collections.singletonMap` 方法将一个包含集合变量的 Map 对象作为参数传入，`SqlSession` 会自动将 Map 中的键值对传递给 Mapper.xml 中的 `foreach` 元素。在执行完 `update` 方法后，需要手动调用 `commit` 方法提交事务，否则更新操作不会生效。