mybatis collection 子查询
时间: 2023-11-07 14:15:13 浏览: 49
MyBatis allows you to perform collection subqueries, which can be useful when you need to retrieve related data for a collection of records. Collection subqueries are typically used in scenarios where you have a one-to-many or many-to-many relationship between tables.
To implement a collection subquery in MyBatis, you can use the `<collection>` element within your result mapping. Here's an example:
```xml
<resultMap id="userMap" type="User">
<id property="id" column="user_id" />
<result property="name" column="user_name" />
<collection property="orders" ofType="Order">
<id property="id" column="order_id" />
<result property="product" column="product_name" />
</collection>
</resultMap>
```
In this example, we have a `User` object that has a collection of `Order` objects. The `User` object has an `id` and a `name`, while the `Order` object has an `id` and a `product`. The `collection` element is used to define the mapping for the collection property (`orders`) of the `User` object.
To perform the collection subquery, you can use the `<select>` element with a nested `<collection>` element in your SQL statement. Here's an example:
```xml
<select id="getUserWithOrders" resultMap="userMap">
SELECT u.id AS user_id, u.name AS user_name,
o.id AS order_id, o.product AS product_name
FROM users u
INNER JOIN orders o ON u.id = o.user_id
WHERE u.id = #{userId}
</select>
```
In this example, we join the `users` and `orders` tables on the `user_id` column and retrieve the relevant columns for both the `User` and `Order` objects. The `<collection>` element is used to map the query results to the collection property (`orders`) of the `User` object.
You can then use this mapping and SQL statement in your MyBatis mapper interface to retrieve a `User` object with its associated `Order` objects.
I hope this explanation helps! Let me know if you have any further questions.
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