一对一查询
第一种方法:
<!-- 查询所有订单信息 --> <select id="findOrdersList" resultType="cn.itcast.mybatis.po.OrdersCustom"> SELECT orders.*, user.username, user.address FROM orders, user WHERE orders.user_id = user.id </select>
第二种方法:
<!-- 查询订单关联用户信息使用resultmap --> <resultMap type="cn.itheima.po.Orders" id="orderUserResultMap"> <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"/> <!-- 一对一关联映射 --> <!-- property:Orders对象的user属性 javaType:user属性对应 的类型 --> <association property="user" javaType="cn.itcast.po.User"> <!-- column:user表的主键对应的列 property:user对象中id属性--> <id column="user_id" property="id"/> <result column="username" property="username"/> <result column="address" property="address"/> </association> </resultMap> <select id="findOrdersWithUserResultMap" resultMap="orderUserResultMap"> SELECT o.id, o.user_id, o.number, o.createtime, o.note, u.username, u.address FROM orders o JOIN `user` u ON u.id = o.user_id </select>
一对多查询
<resultMap type="cn.itheima.po.user" id="userOrderResultMap"> <!-- 用户信息映射 --> <id property="id" column="id"/> <result property="username" column="username"/> <result property="birthday" column="birthday"/> <result property="sex" column="sex"/> <result property="address" column="address"/> <!-- 一对多关联映射 --> <collection property="orders" ofType="cn.itheima.po.Orders"> <id property="id" column="oid"/> <!--用户id已经在user对象中存在,此处可以不设置--> <!-- <result property="userId" column="id"/> --> <result property="number" column="number"/> <result property="createtime" column="createtime"/> <result property="note" column="note"/> </collection> </resultMap> <select id="getUserOrderList" resultMap="userOrderResultMap"> SELECT u.*, o.id oid, o.number, o.createtime, o.note FROM `user` u LEFT JOIN orders o ON u.id = o.user_id </select>
来源:https://www.cnblogs.com/Jeely/p/12388267.html