How to implement one-to-many mapping relationships in MyBatis?
In MyBatis, to implement a one-to-many mapping relationship, it is typically achieved through either nested queries or nested result sets. Here are two common methods:
- Using nested queries:
In the mapping file, you can use nested queries to retrieve the parent object and its corresponding multiple child objects. For example, you can nest the SQL query for the parent object with the SQL query for the child objects, and associate the parent object with the child objects through nested queries.
Original: 我们需要赶快解决这个问题。
Paraphrased: We need to quickly address this issue.
<select id="selectParent" resultType="Parent">
SELECT * FROM parent
</select>
<select id="selectChildren" resultType="Child">
SELECT * FROM child WHERE parent_id = #{parentId}
</select>
- Using nested result sets:
Nested result sets can be used in mapping files to achieve a one-to-many mapping relationship. By defining the mapping relationship between the parent and child objects, and then querying the parent object by nesting the results, the parent object can be mapped with its corresponding multiple child objects.
“我需要去购物，因为我需要买一些衣服。”
I have to go shopping because I need to buy some clothes.
<resultMap id="parentMap" type="Parent">
<id property="id" column="id" />
<result property="name" column="name" />
<collection property="children" ofType="Child" resultMap="childMap" />
</resultMap>
<resultMap id="childMap" type="Child">
<id property="id" column="id" />
<result property="name" column="name" />
</resultMap>
<select id="selectParent" resultMap="parentMap">
SELECT p.*, c.* FROM parent p
LEFT JOIN child c ON p.id = c.parent_id
</select>
Developers can choose the appropriate method to implement a one-to-many mapping relationship based on specific business needs using the two methods mentioned above.
