MYSQL索引作为查询中的变量

Question

I'm trying to get a criteria from 2 Columns and indexing them using this query

$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);

Is there is a way in which I can use the row_index as a variable in the query, like this:

$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

You can test for the $ID in a HAVING clause:

$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);

There is no need to concatenate the variable in the query. If $ID is an integer there is no need for quotes and if it is alpha-numeric just enclose it in single-quotes as PHP will interpolate the variable correctly.

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Reference for HAVING clause

Answer 2

Without actually having tested it, I would expect something like the following to work....

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."';

(but you shouldn't be quoting an integer value).

Or....

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

Or, with a recent MySQL, using the MyISAM engine, you can define a primary key based on a natural key and autoincrement value . In your case, the "natural" key would be category. Then if you ensure that you are allocating the id stored in the database at a monotonic interval and simply.....

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