一对一查询配置
<!-- 一对一查询 ，一个订单对应一个用户-->
<select id="findOrdersUser" resultType="com.OrdersCustom" resultMap="OrdersUserResultMap">
SELECT
t1.*,
t2.*
FROM
orders t1
LEFT JOIN
t_user t2
WHERE
t1.user_id=t2.id
</select>
<resultMap type="com.Orders" id="OrdersUserResultMap">
<id column="id" property="id"/>
<result column="user_id" property="userid"/>
<result column="number" property="number"/>
<result column="createtime" property="createTime"/>
<result column="note" property="note"/>
<!-- 订单关联用户 -->
<association property="user" javaType="com.User">
<id column="user_id" property="id"/>
<result column="username" property="username"/>
<result column="sex" property="sex"/>
<result column="address" property="address"/>
</association>
</resultMap>
一对多查询（left join 主要是为了当多方为null时一方可以查）
<!-- 一对多查询，一个订单对应多个订单明细 -->
<select id="findOrdersUser" resultType="com.OrdersCustom" resultMap="OrdersUserResultMap">
SELECT
t1.*,
t2.*
FROM
orders t1
LEFT JOIN
t_user t2
WHERE
t1.user_id=t2.id
</select>
<resultMap type="com.Orders" id="ordersAndOrderDetailResultMap">
<id column="id" property="id"/>
<result column="user_id" property="userid"/>
<result column="number" property="number"/>
<result column="createtime" property="createTime"/>
<result column="note" property="note"/>
<!-- 关联订单详细 -->
<collection property="orderdetails" ofType="com.OrderDetail">
<id column="orderdetail_id" property="id"/>
<result column="items_id" property="itemsId"/>
<result column="items_num" property="itemsNum"/>
<result column="orders_id" property="ordersId"/>
</collection>
</resultMap>
多对多查询
<!-- 多对多查询，查询用户以及用户购买的商品信息，一个用户对应多个订单，一个订单对应多个订单明细，一个订单明细对应一个商品 -->
<select id="findUserAndItemsResultMap" resultMap="userAndItemsResultMap">
SELECT
t1.*,
t2.username,
t2.sex,
t2.address,
t3.id orderdetail_id,
t3.items_id,
t3.items_num,
t3.orders_id,
t4.itemsname items_name,
t4.detail items_detail,
t4.price items_price
FROM
orders t1,
t_user t2,
orderdetail t3,
items t4
WHERE t1.user_id = t2.id AND t3.orders_id=t1.id AND t3.items_id = t4.id
</select>
<resultMap type="com.User" id="userAndItemsResultMap">
<!-- 用户信息 -->
<id column="user_id" property="id"/>
<result column="username" property="username"/>
<result column="sex" property="sex"/>
<result column="address" property="address"/>
<!-- 一个用户对应多个订单 -->
<collection property="ordersList" ofType="com.Orders">
<id column="id" property="id"/>
<result column="user_id" property="userid"/>
<result column="number" property="number"/>
<result column="createtime" property="createTime"/>
<result column="note" property="note"/>
<!-- 一个订单对应多个订单明细 -->
<collection property="orderdetails" ofType="com.OrderDetail">
<id column="orderdetail_id" property="id"/>
<result column="items_id" property="itemsId"/>
<result column="items_num" property="itemsNum"/>
<result column="orders_id" property="ordersId"/>
<!-- 一个订单明细对应一个商品 -->
<association property="items" javaType="com.Items">
<id column="items_id" property="id"/>
<result column="items_name" property="itemsName"/>
<result column="items_detail" property="detail"/>
<result column="items_price" property="price"/>
</association>
</collection>
</collection>
</resultMap>
