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.