如何确保随机选择的两行彼此不同? [英] How can I make sure that two rows selected at random are different from one another?

问题描述

我有一个表 photos 里面有很多照片，我需要随机选择两张:

I have a table photos with many photos in it and I need to select two at random:

在getnew.php

$result = mysqli_query($conn,"SELECT * FROM photos ORDER BY rand() LIMIT 1");
$result2 = mysqli_query($conn,"SELECT * FROM photos ORDER BY rand() LIMIT 1");

  $row = $result->fetch_assoc();
  $img1link = $row['link'];
  // more stuff from $row

  $row2 = $result2->fetch_assoc();
  $img2link = $row2['link'];
  // more stuff from $row2

但是我需要防止它两次选择同一张照片(所选照片必须不同)，即$img1link 不应= $img2link.然后我需要在另一个文件中使用 $.getJSON 检索数据，使用 getnew.php 末尾的数组.

However I need to prevent it from selecting the same photo twice (the selected photos must be different), i.e. $img1link should not = $img2link. I then need to retrieve the data using $.getJSON in another file, using an array at the end of getnew.php.

getnew.php 末尾的数组:

echo json_encode(array('img1'=>$img1link,'img2'=>$img2link, ...(etc)... ));

如何确保所选照片在变量存储在数组中时有所不同?我试图创建一个 if/else 语句，但并没有真正理解我在做什么.

How can I make sure the selected photos are different by the time the variable is stored in the array? I tried to create an if/else statement but didn't really understand what I was doing.

推荐答案

你可以只执行一次，但可以改为执行两次，这样你就永远不会选择同一行:

You can just execute once but get two instead so that you'll never pick the same row:

$result = mysqli_query($conn,"SELECT * FROM photos ORDER BY rand() LIMIT 2");
$row = $result->fetch_assoc();
$row2 = $result->fetch_assoc();
// invoke `->fetch` twice to get the first and second row
$img1link = $row['link'];
$img2link = $row2['link'];

旁注:请注意 ORDER BY rand() 子句，因为它在大型数据集上会很慢.您可以使用@Bill Karwin 出色的answer

Sidenote: Be careful of that ORDER BY rand() clause since it'll be slow on large data sets. You can use an alternative with @Bill Karwin's great answer

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