从MySQL表中选择最大编号无效

Question

I've looked this up and I've yet to figure this out. This is part of my table.

What I'm trying to do is, grab the biggest number in the tradeNum column. What I have so far is:

$sql = "SELECT MAX(tradeNum) FROM trades";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo "num: " . $row["tradeNum"] . "<br>";
    }
} else {
    echo "0 results";
}

While this doesn't echo "0 results," it echos nothing. I'm still new to mysql, but shouldn't this work? I took most of the code from here if it helps. Sorry if I'm vague, let me know if I need to clear something up.

Answer 1

您查询的应该是

$sql = "SELECT MAX(tradeNum) AS tradeNum FROM trades";

Answer 2

$sql = "SELECT MAX(tradeNum) tradeNum FROM trades"; // here use tradeNum as alias
$result = $conn->query($sql);
echo $result[0]['tradeNum'];

Answer 3

由于您使用的是tradeNum列来获取数据，因此您还可以使用带有limit order by

SELECT tradeNum FROM trades order by tradeNum desc limit 1