Question

我希望该表能够使用右连接从每个主题返回最多2个回复。我可以知道我该怎么做？

<?php
include_once "connect.php";
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password); 

$form=$_GET;
$trade=$form['tradetype'];
$metal=$form['metal'];
$amount=$form['amount'];
$date=$form['date'];

$stmt = $conn->query("SELECT Discount FROM Contracts WHERE Trade='$trade' AND Metal='$metal' AND Amount='$amount' AND ExpiryDate='$date'");
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) { 
   echo ($row['Discount']); 
} 
?>

Answer 1

这个前N个查询怎么样：

select tid, rid from(
    select t.tid, r.rid,
           case when @tid is not null and @tid != t.tid then @rn := 0 else null end reset_rn,
           @tid := t.tid tid_change,
           @rn := @rn + 1 rn
    from   topic t
    left join reply r on t.tid = r.tid
    join (select @rn := 0, @tid := null) rn
    order by t.tid, r.rid
) q
where rn < 3;

保留2行的关键是内部查询中的order by。

查询以从每个主题获得最多2个回复

1 个答案: