无法解析为变量 - JDBC

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我正在使用Java和SQL库处理Oracle数据库。我试图计算平均值偏离平均值一个标准偏差的学生，添加一列来标记这些学生，并计算有多少学生符合条件并将其添加到新表中。我收到一个错误，指出“allAvgs”变量和“theSD”变量“无法解析为变量”。错误发生在我的if语句中，但我已经实例化了这些变量。为什么我会得到这个错误？任何帮助表示赞赏。无法解析为变量 - JDBC

try{ 
     Statement stOne, stTwo, stThree, stFour; 
     String SelectAverage = "SELECT MEAN FROM STUDENTS"; 
      ResultSet rsOne = stOne.executeQuery(SelectAverage); 

    String TotalAverage = "SELECT Avg(MEAN) AS averages FROM STUDENTS"; 
      ResultSet rsTwo = stTwo.executeQuery(TotalAverage);  

      String student_stan_dev = "SELECT STDEV(MEAN) AS standardDeviation FROM STUDENTS"; 
      ResultSet rsThree = stThree.executeQuery(student_stan_dev); 

      int onesdMean = 1; 


      //Loop Duration_Sec column 
      while(rsOne.next()){ 

      //Convert values into float values 
      float allAvgs = rsOne.getFloat("MEAN"); 
      float totalAvg = rsTwo.getFloat("averages"); 
      float StDev = rsThree.getFloat("standardDeviation"); 

      float theSD = allAvgs - (onesdMean * StDev); 
      } 

     String flaggedStudents = "ALTER TABLE STUDENTS ADD FlaggedStudents INT"; 
     ResultSet rsFour = stFour.executeUpdate(flaggedStudents); 

      if(allAvgs >= theSD){ 
       String FlagHint = "INSERT INTO STUDENTS.FlaggedStudents VALUES('1')"; 
       st.executeUpdate(FlagHint); 
      } 

     String countInstances = "SELECT STUDENTS.NAME, STUDENTS.FlaggedStudents" + 
       "COUNT(*)OVER(PARTITION BY STUDENTS) AS cnt FROM STUDENTS"; 
      st.executeQuery(countInstances); 


     st.executeUpdate("CREATE TABLE IF NOT EXISTS StudentCount" + 
     "(NAME INT , cnt INT)"); 

     String insertVals = String.format("INSERT INTO StudentCount" + 
       "(NAME , cnt INT") + 
       " VALUES ('%s','%s')"); 

     st.execute(insertVals);enter code here

来源

2013-07-24 user2554121

您不能在声明它的块之外使用'theSD'。它超出了范围。 –

谢谢你们回答我的问题，它工作！我感谢您的帮助。 – user2554121

，因为它们是while(){...}环路内声明和限定于while()环路的块范围。您需要在循环外部声明它们以扩展其范围。

// you need to declare the variables outside 
while(rsOne.next()){ 
    // this defines a scope , anything declared here is invisible outside 
    // this scope 

    float allAvgs = rsOne.getFloat("MEAN"); 
    float totalAvg = rsTwo.getFloat("averages"); 
    float StDev = rsThree.getFloat("standardDeviation"); 
    float theSD = allAvgs - (onesdMean * StDev); 
} 
...... 
...... 
// compiler has no idea about allAvgs and theSD here 
if(allAvgs >= theSD){ 
    String FlagHint = "INSERT INTO STUDENTS.FlaggedStudents VALUES('1')"; 
    st.executeUpdate(FlagHint); 
}

来源

2013-07-24 18:46:53 NINCOMPOOP

你已经宣布theSD的while循环块内。

//Loop Duration_Sec column 
while(rsOne.next()){ 

    ... 
    float theSD = allAvgs - (onesdMean * StDev); 
}

但试着在它后面使用它。

... 

if(allAvgs >= theSD){ // theSD doesn't exist in this scope 
    String FlagHint = "INSERT INTO STUDENTS.FlaggedStudents VALUES('1')"; 
    st.executeUpdate(FlagHint); 
}

它不存在于while循环范围之外。您需要在while循环外声明（并初始化）它（以及所有其他所需）。 考虑如果while循环评估为false并且未运行，会发生什么情况。

来源

2013-07-24 18:48:44

无法解析为变量 - JDBC

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