JSP JDBC 查詢 COUNT(*) 正確獲取結果集數值

錯誤代碼

ResultSet rst = stt.executeQuery("select count(*) from t_students");
out.print(rst.getString("")); // ❌ 空字串無效

正確方式

1. 別名 + getInt/getLong (推薦)：

   ResultSet rst = stt.executeQuery("select count(*) as cnt from t_students");
   if (rst.next()) {
       int total = rst.getInt("cnt"); // 或 getLong
       out.print(total);
   }
   

2. 索引存取：

   if (rst.next()) out.print(rst.getInt(1));
   

3. getString 索引：

   out.print(rst.getString(1));
   

完整範例

<%
Connection conn = null;
try {
    Class.forName("com.mysql.jdbc.Driver");
    conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "user", "pass");
    Statement st = conn.createStatement();
    ResultSet rs = st.executeQuery("SELECT COUNT(*) AS total FROM t_students");
    if (rs.next()) {
        out.println("學生總數: " + rs.getInt("total"));
    }
} catch (Exception e) {
    e.printStackTrace();
} finally {
    try { if (conn != null) conn.close(); } catch (Exception ignored) {}
}
%>

關鍵點

• 聚合查詢**必須給別名**或用索引 1 取值。
• 永遠用 if (rs.next()) 而非 while，因 COUNT 只返回一行。
• 資源在 finally 關閉，防洩漏。