MYSQL指数在查询中的变量

Question

我试图从2列得到一个标准和索引他们使用此查询

$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为变量的方法在查询中，像这样：

$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条款测试为$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); 

在查询中不需要连接变量。如果$ID是一个整数，则不需要引号，并且如果它是字母数字，只需将它括在单引号中，因为PHP将正确插入变量。

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Reference for HAVING clause

Answer 2

而无需实际测试它，我希望类似如下的工作....

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."'; 

（但你不应该引用整数值）。

或者....

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

或者，与最近的MySQL，使用MyISAM引擎，可以define a primary key based on a natural key and autoincrement value。在你的情况下，“自然”键将是类别。然后，如果你确保你是在单调区间分配数据库中的存储的ID ，只是.....

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