Mapped Statements collection already contains value

今天来分享一下让人让我有点抓脑的问题！！

这一阵子都在自学Mybatis,昨天学的多表查询，多对一的关系查询中出现了一个报错，其实也是很简单的问题，但稍不注意就会让人抓耳挠腮。

在数据库的字段名中有些事下划线命名，而对应的实体类一般会是驼峰命名，解决字段名和属性名不一致的方法有三种：

a->:为字段起别名

b->:

设置全局变量，将自动映射为驼峰 <setting name="mapUnderscoreToCamelCase" value="true"/>

c->:

设置resultMap

<resultMap id="empResultMap" type="Emp"><id property="eid" column="eid"></id><result property="empName" column="emp_name"></result><result property="age" column="age"></result><result property="sex" column="sex"></result><result property="email" column="email"></result>
</resultMap>

这里要注意的是：在mpMApper.xml写查询sql语句时，同一个方法的resultMap的值不能写多个，否则会出现Mapped Statements collection already contains value

而且，每一条查询语句的resultMap都得有相对应的唯一的id,比如：

<resultMap id="getAssociationEmpAndDept" type="Emp">

这样会导致查找不到相应的resultMap ：

Result Maps collection does not contain value for com.atguigu.mybatis.mapper.EmpMapper.getAssociationEmpAndDept

出现这样的报错一定要去仔细查找自己resultMap 设置得对应的id。

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