java.sql.SQLException: 列 'Max(category_id' 未找到)

Question

Here is my code. It gives me an exception error says "java.sql.SQLException: Column 'Max(category_id' not found.". Please help. Thanks in advance.

enter code here

public class Category extends javax.swing.JFrame {

/**
 * Creates new form Category
 */
public Category() {
    initComponents();
    DisplayTable();
    autoID();
}


//Display Table
private void DisplayTable() {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/inventory?useTimezone=true&serverTimezone=UTC", "root", "ichigo197328");
        String sql = "SELECT * FROM category";
        PreparedStatement pstmt = conn.prepareStatement(sql);
        ResultSet rs = pstmt.executeQuery();
        jTable1.setModel(DbUtils.resultSetToTableModel(rs));
    }
    catch(Exception e) {
        JOptionPane.showMessageDialog(null, e);
    }
}

public void autoID() {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/inventory?useTimezone=true&serverTimezone=UTC", "root", "ichigo197328");
        Statement s = conn.createStatement();

        ResultSet rs = s.executeQuery("SELECT Max(category_id) from category");
        rs.next();

        rs.getString("Max(category_id)");

        if(rs.getString("Max(category_id") == null) {
            CategoryIDField.setText("C0001");
        }
        else {
            Long id = Long.parseLong(rs.getString("Max(category_id").substring(2, rs.getString("Max(category_id").length()));
            id++;
            CategoryIDField.setText("C0" + String.format("%03d", id ));
        }
    }
    catch(ClassNotFoundException e) {
        Logger.getLogger(Category.class.getName()).log(Level.SEVERE, null, e);
    }
    catch(SQLException e) {
        Logger.getLogger(Category.class.getName()).log(Level.SEVERE, null, e);
    }
}

Answer 1

The column has a default name but it isn't the same as the function, the easiest option would be to change all

rs.getString("Max(category_id)");

to

rs.getString(1);

Alternatively, name the column in your query. Like,

ResultSet rs = s.executeQuery("SELECT Max(category_id) AS FRED from category");

then use

rs.getString("FRED");

for example. Finally, you should be using getInt or getLong if the column is of those types (which I suspect because you are taking the MAX).

Answer 2

I think in the line

    if(rs.getString("Max(category_id") == null) {
        CategoryIDField.setText("C0001")

the quote should be after the round bracket.

Answer 3

使用 alisa select Max(category_id) as xxx from category