如何將for語句的結果放入表中？

Question

將for聲明的結果放入PostgreSQL表中的最佳方式是什麼？

例子：

int size = 5; 
try { 
    for (int i = 1; i <= size; i++) { 
     for (int j = 1; j <= size; j++) { 
      ??? x = (i * j); 

      String sql = "INSERT INTO multiplication" + 
         "VALUES (???)"; 
      ResultSet rs = stmt.executeQuery(sql); 
     } 
    } 
} 

Answer 1

我建議你使用準備好的語句：

int size = 5; 
String strSQL = "insert into multiplication values (?)"; 
/* 
* I am assuming your table has only one column. 
* If it has more columns, you need to specify the column where you want 
* to insert the value: 
*  "insert into multiplication (col1) values (?)" 
* 
* The question mark is a place-holder for the value you are about to insert. 
* 
* I am also assuming that you have a Connection object named "conn". 
*/ 
try(PreparedStatement ps = conn.prepareStatement(strSQL)) { 
    for(int i = 1; i <= size; i++) { 
     for(int j = 1; j <= size; j++) { 
      /* 
      * You are multiplying integers, so the result is also an integer 
      */ 
      int x = i * j; 
      /* 
      * Assign the value to the prepared statement 
      */ 
      ps.setInt(1, x); 
      ps.execute(); // The statement is executed HERE 
     } 
    } 
} catch(SQLException e) { 
    // Exception handling code 
} 

預處理語句有一定的優勢：

1. 您可以編寫SQL指令只有一次，並使用了很多次。您只需輸入佔位符的索引（從1開始）並分配適當的值。對於大多數SQL數據類型，有setXXX()方法（如果需要的話可以使用setNull()方法）。
2. 預處理語句有助於防止SQL injection attacs（（關於什麼是SQL注入攻擊的風險，一個有趣的例子，檢查xkcd: Exploits of a Mom）。
3. 如果你有大量的指令，你可以在一個批次中執行它們。

批次例如：

int size = 5; 
String strSQL = "insert into multiplication values (?)"; 
try(PreparedStatement ps = conn.prepareStatement(strSQL)) { 
    for(int i = 1; i <= size; i++) { 
     for(int j = 1; j <= size; j++) { 
      int x = i * j; 
      ps.setInt(1, x); 
      ps.addBatch(); // The statement is added to the batch (but not executed yet) 
     } 
    } 
    ps.executeBatch(); // All the instructions in the batch are executed HERE 
} catch(SQLException e) { 
    // Exception handling code 
} 

---

個參考文獻：

• The Java Tutorials: JDBC - Prepared statements
• Java API: PreparedStatement

Answer 2

你可以試試這個。

int size = 5; 
try { 
    for (int i = 1; i <= size; i++) { 
     for (int j = 1; j <= size; j++) { 
      String x = String.valueOf(i * j); 

      String sql = "INSERT INTO multiplication" + 
        "VALUES (" + x + ")"; 
      ResultSet rs = stmt.executeQuery(sql); 
     } 
    } 
}