MYSQL指數在查詢中的變量

Question

我試圖從2列得到一個標準和索引他們使用此查詢

$query1 = "SET @row_num = 0"; 
$query2 = "SELECT *, @row_num := @row_num + 1 as row_index FROM gift 
      WHERE Category = '0' AND ID ='".$ID."' 
      ORDER BY ID ASC;"; 
mysqli_query($conn, $query1); 

$retrieve = mysqli_query($conn, $query2); 

是否存在，我可以使用ROW_INDEX爲變量的方法在查詢中，像這樣：

$query1 = "SET @row_num = 0"; 
$query2 = "SELECT *, @row_num := @row_num + 1 as row_index FROM gift 
      WHERE Category = '0' AND row_index ='".$ID."' 
      ORDER BY ID ASC;"; 
mysqli_query($conn, $query1); 

$retrieve = mysqli_query($conn, $query2); 

Answer 1

您可以在HAVING條款測試爲$ID：

$query1 = "SET @row_num = 0"; 
$query2 = "SELECT *, @row_num := @row_num + 1 as row_index FROM gift 
      WHERE Category = '0' 
      HAVING row_index = $ID 
      ORDER BY ID ASC;"; 
mysqli_query($conn, $query1); 

$retrieve = mysqli_query($conn, $query2); 

在查詢中不需要連接變量。如果$ID是一個整數，則不需要引號，並且如果它是字母數字，只需將它括在單引號中，因爲PHP將正確插入變量。

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Reference for HAVING clause

Answer 2

而無需實際測試它，我希望類似如下的工作....

SELECT ilv.* 
FROM (
    SELECT gift.*, @row_num := @row_num + 1 as row_index 
    FROM gift 
    WHERE Category = '0' 
    ORDER BY ID ASC 
) ilv 
WHERE row_index ='".$ID."'; 

（但你不應該引用整數值）。

或者....

SELECT * 
FROM gift 
WHERE Category = '0' 
ORDER BY ID ASC 
LIMIT $ID, 1; 

或者，與最近的MySQL，使用MyISAM引擎，可以define a primary key based on a natural key and autoincrement value。在你的情況下，「自然」鍵將是類別。然後，如果你確保你是在單調區間分配數據庫中的存儲的ID ，只是.....

SELECT * 
FROM gift 
WHERE category='0' 
AND id=($ID * $interval);