How can we perform a join query operation in MyBatis?

When performing join operations in MyBatis, there are various ways to achieve it, such as using tags in the Mapper XML file and annotations, or utilizing annotation-based join queries.

Here is an example of conducting a join query using a Mapper XML file:

1. Labels and tags, as shown below:

<resultMap id="userMap" type="User">
    <id property="id" column="user_id" />
    <result property="username" column="username" />
    <result property="email" column="email" />
</resultMap>

<resultMap id="orderMap" type="Order">
    <id property="id" column="order_id" />
    <result property="orderNo" column="order_no" />
    <result property="amount" column="amount" />
    <association property="user" javaType="User" resultMap="userMap" />
</resultMap>

1. In SQL statements, use the JOIN operation to connect two tables and map the query results to two entity classes as shown below:

<select id="findOrderById" parameterType="int" resultMap="orderMap">
    SELECT o.*, u.user_id, u.username, u.email
    FROM orders o
    INNER JOIN users u
    ON o.user_id = u.user_id
    WHERE o.order_id = #{orderId}
</select>

1. Define a query method in the corresponding Mapper interface and call the corresponding SQL statement in the method as shown below:

public interface OrderMapper {
    Order findOrderById(int orderId);
}

By following the above steps, you can perform join operations in MyBatis. Alternatively, you can also use annotation to perform join queries, for more details please refer to the official MyBatis documentation.