mybatis中：returned more than one row, where no more than one was expected.异常

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.executor.ExecutorException: Statement returned more than one row, where no more than one was expected.
    at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:75) ~[MyBatisExceptionTranslator.class:1.2.2]
    at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:371) ~[SqlSessionTemplate$SqlSessionInterceptor.class:1.2.2]

Caused by: org.apache.ibatis.executor.ExecutorException: Statement returned more than one row, where no more than one was expected.
    at org.apache.ibatis.executor.ResultExtractor.extractObjectFromList(ResultExtractor.java:58) ~[ResultExtractor.class:3.2.8]
    at org.apache.ibatis.executor.loader.ResultLoader.loadResult(ResultLoader.java:71) ~[ResultLoader.class:3.2.8]

由于查找出了多个对象，一般查找的那个字段是数据有重复。

程序期望返回不超过一行数据，但实际返回了多于一行的数据。

比如sql语句的返回类型是非集合类型，但返回了多行数据。