MYSQL Index as a variable in a query - php

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.
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);