rg.apache.ibatis.binding.BindingException: Mapper method 'com.dao.Cameao.getOnlineDayRation attempted to return null from a method with a primitive return type (float)

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异常展示如下：

org.apache.ibatis.binding.BindingException: Mapper method 'com.dao.Cameao.getOnlineDayRation attempted to return null from a method with a primitive return type (float).
    at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:)
    at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:)
    at com.sun.proxy.$Proxy36.getOnlineDayRation(Unknown Source)
    at com.allcam.rss.modules.camerastatistics.impl.CameraOnlineDayStatisticsServiceImpl.getOnlineDayRation(CameraOnlineDayStatisticsServiceImpl.java:)
    at com.allcam.rss.controller.user.UserController.index(UserController.java:)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:)
    at java.lang.reflect.Method.invoke(Method.java:)
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at com.allcam.rss.system.filter.SystemFilter.doFilter(SystemFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:)
    at java.lang.Thread.run(Thread.java:)

异常解析：

由于AVG求平均数时，可能为0导致报错，解决方法：

修改sql，使用IFNULL函数即可解决问题：

SELECT IFNULL(AVG(ONLINERATION),0.00) TBL_CAMERA_ONLINE_STAT_YYYYMM WHERE statdate=#{dayOnlineTime}

即可解决问题。