Mybatis框架基础入门(七)--关联查询

1.一对一查询

1.1 使用resultType接收查询结果

修改pojo类

public class OrderUser extends order {
   private String username;
   private String address;
   ...此处省略get set方法
}

<!-- 查询订单，同时包含用户数据 -->
<select id="queryOrderUser" resultType="orderUser">
	SELECT
	o.id,
	o.user_id
	userId,
	o.number,
	o.createtime,
	o.note,
	u.username,
	u.address
	FROM
	`order` o
	LEFT JOIN `user` u ON o.user_id = u.id
</select>

1.2 使用resultMap接收查询结果

修改pojo类

public class Order{
   private int id;
   private Integer userId;
   private String number;
   private User user;
   ..此处省略get set方法
}

<resultMap type="order" id="orderUserResultMap">
	<id property="id" column="id" />
	<result property="userId" column="user_id" />
	<result property="number" column="number" />

	<!-- association ：配置一对一属性 -->
	<!-- property:order里面的User属性名 -->
	<!-- javaType:属性类型 -->
	<association property="user" javaType="user">
		<!-- id:声明主键，表示user_id是关联查询对象的唯一标识-->
		<id property="id" column="user_id" />
		<result property="username" column="username" />
		<result property="address" column="address" />
	</association>

</resultMap>

<!-- 一对一关联，查询订单，订单内部包含用户属性 -->
<select id="queryOrderUserResultMap" resultMap="orderUserResultMap">
	SELECT
	o.id,
	o.user_id,
	o.number,
	u.username,
	u.address
	FROM
	`order` o
	LEFT JOIN `user` u ON o.user_id = u.id
</select>

2. 一对多查询

案例：查询所有用户信息及用户关联的订单信息。

用户信息和订单信息为一对多关系。

修改pojo类

public class User{
   private String username;
   private String address;
   private List<Order> orders;
   ..此处省略get set方法
}

<resultMap type="user" id="userOrderResultMap">
	<id property="id" column="id" />
	<result property="username" column="username" />
	<result property="address" column="address" />

	<!-- 配置一对多的关系 -->
	<collection property="orders" javaType="list" ofType="order">
		<!-- 配置主键，是关联Order的唯一标识 -->
		<id property="id" column="oid" />
		<result property="number" column="number" />
	</collection>
</resultMap>

<!-- 一对多关联，查询订单同时查询该用户下的订单 -->
<select id="queryUserOrder" resultMap="userOrderResultMap">
	SELECT
	u.id,
	u.username,
	u.address,
	o.id oid,
	o.number,
	FROM
	`user` u
	LEFT JOIN `order` o ON u.id = o.user_id
</select>