Question

我创建了这段代码，允许我计算表格中的行数。但是，我无法返回计数的数字，并显示“无法从结果类型为void的方法返回值”。有人能告诉我'我的错误在哪里？非常感谢！

public void num() throws Exception {
  try {
      // This will load the MySQL driver, each DB has its own driver
      Class.forName("com.mysql.jdbc.Driver");
      // Setup the connection with the DB
      connect = DriverManager.getConnection("jdbc:mysql://localhost/testdb?"
      + "user=root&password=");

      // Statements allow to issue SQL queries to the database
      statement = connect.createStatement();
      resultSet = statement.executeQuery("select * from testdb.emg");
      int count = 0;
      while (resultSet.next()) {
        count++;
      }  
      return count;
  } catch (Exception e) {
  }

Answer 1

尝试以下代码

 public int num() throws Exception {
 try {
 // This will load the MySQL driver, each DB has its own driver
 Class.forName("com.mysql.jdbc.Driver");
 // Setup the connection with the DB
 connect = DriverManager.getConnection("jdbc:mysql://localhost/testdb?"
 + "user=root&password=");

 // Statements allow to issue SQL queries to the database
 statement = connect.createStatement();
 resultSet = statement.executeQuery("select count(*) from testdb.emg");

 while (resultSet.next()) {
 return resultSet.getInt(1);
 }
} catch (Exception e) {
}

以下是错误

public void num() throws Exception {

应该是

public int num() throws Exception {
要计算总行数，您应使用查询select count(*) from testdb.emg

让我知道任何问题。

Answer 2

更改

public void num() throws Exception {

到

public int num() throws Exception {

您正在从count类型的变量int返回值，因此该方法的返回类型也应为int。

您还应确保在代码的每个执行路径中都有return语句，包括catch块中的异常处理程序（或者您将收到“缺少返回语句”错误消息）。但是，最好避免捕获所有异常的catch语句（如您的）。此外，忽略（即不处理）catch块中的异常通常会导致难以诊断问题，这是一种不好的做法。

代码还有其他问题：除count外，没有声明任何变量。

请注意，您可以使用以下SQL语句直接获取行数：

select count(*) from testdb.emg

这可以避免将表testdb.emg中的所有数据发送到您的应用程序，并且对于大表来说要快得多。

Answer 3

如何在java中获取count（*）mysql数据表。试试吧：

   public int getRowNumber(){

   int numberRow = 0;
   Connection mysqlConn = DriverManager.getConnection(HOST, USER_ID, PASSWORD);

try{
    mysqlConn.getConnection();
    String query = "select count(*) from dataTable";
    PreparedStatement st = mysqlConn.preparedStatement(query);
    ResultSet rs = st.executeQuery();
    while(rs.next()){
        numberRow = rs.getInt("count(*)");
    }
}catch (Exception ex){
    System.out.println(ex.getMessage());
}
return numberRow;
}

Answer 4

public void num() throws Exception {

应该是

public int num() throws Exception {

Answer 5

我用Fahim Parker的答案做了些改动

`

public int num() throws Exception {
 try {
 Class.forName("com.mysql.jdbc.Driver");
 connect = DriverManager.getConnection("jdbc:mysql://localhost/testdb?"
 + "user=root&password=");

 statement = connect.createStatement();
 resultSet = statement.executeQuery("<your query statement>");

 resultSet.last(); //go to last row;
 return resultSet.getRow(); //get row number which is equal to rows count

} catch (Exception e) {
}

`

java mysql计算行数

5 个答案: