一對多關系：一是用戶，多是訂單

實體類User

public class User {private Integer id;private String name;private Integer age;private List<UserOrder> orders;}

實體類UserOrder

public class UserOrder {private Integer id;private String name;private Integer userId; }

使用嵌套結果來查詢：相當于使用一條sql語句關聯多張表查詢出所有結果，然后根據映射給訂單類屬性賦值

<!--1.嵌套結果--> <select id="selectUser" resultMap="selectUser1">select u.id,u.name,u.age,o.id as order_id,o.name as order_name from t_p_user u left join t_p_user_order oon u.id = o.user_id and u.id = #{id}</select><resultMap id="selectUser1" type="com.seakoon.model.User"><id column="id" property="id" /><result column="name" property="name"/><result column="age" property="age"/><collection property="orders" ofType="com.seakoon.model.UserOrder" column="id"><id column="order_id" property="id" /><result column="order_name" property="name" /><result column="id" property="userId" /></collection></resultMap>

使用嵌套查詢：查詢訂單數據時新用一條sql來查。參數是column屬性的值，如果多參數的話：使用
column=“{property1 = column1,property2 = column2…}”

<!--2.嵌套查詢--><select id="selectUserNew" resultMap="selectUser2">select id,name,age from t_p_user where id = #{id}</select><resultMap id="selectUser2" type="com.seakoon.model.User"><id column="id" property="id" /><result column="name" property="name"/><result column="age" property="age"/><collection property="orders" ofType="com.seakoon.model.UserOrder" column="id" javaType="java.util.ArrayList"select="selectOrderById"><id column="order_id" property="id" /><result column="order_name" property="name" /><result column="id" property="userId" /></collection></resultMap><select id="selectOrderById" resultType="com.seakoon.model.UserOrder">select id,name,user_id as userId from t_p_user_order where user_id = #{id}</select>

總結

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