如何判断SQL查询语句执行后ResultSet是否返回数据或为空？

Hey there! Let's tackle your problem step by step, fix the code issues, and cover some important best practices along the way.

The Core Fix: Checking for ResultSet Data #

Right after running executeQuery(), your ResultSet starts positioned before the first row. To verify if there's any data returned, you just need to use the next() method:

• If next() returns true, it means the cursor successfully moved to a valid row with data.
• If it returns false, there are no rows in the ResultSet.

Updated & Fixed Code #

Here's your code with the correct check, syntax fixes (like the misplaced parentheses after else), and a critical security improvement to avoid SQL injection:

try{ 
    // Use PreparedStatement instead of Statement to prevent SQL injection!
    String sql = "SELECT * from BOOKS WHERE ISBN_No = ?"; 
    PreparedStatement pstm = conn.prepareStatement(sql);
    pstm.setString(1, line); // Safely bind the ISBN value
    
    ResultSet rs = pstm.executeQuery(); 
    
    if(rs.next()) { // This checks if at least one row exists
        // Uncomment below if you need to re-read the result later (cursor moves forward once)
        // rs.beforeFirst();
        displayBookInfo(line); 
    } else { 
        System.out.println("No book found"); 
    } 
    
    // Close resources in reverse order of creation to avoid leaks
    rs.close();
    pstm.close(); 
} catch (SQLException e) { 
    e.printStackTrace(); 
    System.out.println("Failed to search for book with ISBN: " + line ); 
    noException = false; 
}

Key Details to Remember: #

• How next() works: When you first get the ResultSet, the cursor sits at position -1 (before the first row). Calling next() shifts it to the first row (position 0) if it exists, returning true. If no rows exist, it stays at -1 and returns false.
• SQL Injection Risk: Your original code used string concatenation to build the query, which is a major security hazard. PreparedStatement with parameter placeholders (?) separates SQL logic from user input, eliminating this risk entirely.
• Resource Cleanup: Always close ResultSet, Statement/PreparedStatement, and Connection in reverse order of how you created them to prevent resource leaks.

内容的提问来源于stack exchange，提问作者Swagatg Gautam