PHP/MYSQL subquery causing undefined index error - php

Question

I am trying to do a subquery using PDO:
$stmt = $pdo->prepare("SELECT sum(ros_ranking) FROM (SELECT ros_ranking FROM players WHERE teamid = 1 and positionid = 1 ORDER BY ros_ranking ASC LIMIT 1) AS totalrankings");
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);
$num_rows= $stmt->rowCount();
echo "tqr= ".$row['totalrankings'];
I keep getting Notice: Undefined index: totalrankings in ...
ros_ranking is definitely a field with numbers in it in my Players table and a teamid of 1 and positionid of 1 definitely exist.
Does anyone see what I'm doing wrong here? FYI, if I do just the subquery as the main query, it works. So something must be wrong with SELECT sum(ros_ranking) FROM or AS totalrankings but they seem pretty straightforward.

Answer 1

Wrong alias use/position
you need a column name alias for totalrankings (not a table name alias as you have done)
$stmt = $pdo->prepare("SELECT sum(t.ros_ranking) totalrankings
FROM (SELECT ros_ranking
FROM players
WHERE teamid = 1
AND positionid = 1
ORDER BY ros_ranking ASC LIMIT 1) t");
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);
$num_rows= $stmt->rowCount();
echo "tqr= ".$row['totalrankings'];

Answer 2

total_rankings is an alias for the subquery, it's not an alias for the column. You need to put AS totalrankings after the SUM() expression.
$stmt = $pdo->prepare("
SELECT sum(ros_ranking) AS totalrankings
FROM (
SELECT ros_ranking
FROM players
WHERE teamid = 1 and positionid = 1
ORDER BY ros_ranking ASC
LIMIT 1) AS subquery");
But LIMIT 1 in the subquery means you're not going to get a sum of anything. The subquery just returns 1 row, and the sum of one thing is just that value.

Answer 3

Though you can fix it, by giving the selected column the right alias, there is no need to fetch an array if your query returns only one column. Use fetchColumn() instead:
$stmt = $pdo->prepare("SELECT ...");
$stmt->execute();
$totalrankings = $stmt->fetchColumn();
$num_rows= $stmt->rowCount();
echo "tqr= ".$totalrankings;