如何实现SQL语句来创建/或功能

Question

SQL statement question. Let's say I have the following in my mini-table. Let's say the table is "images."

ID  |  file_name   |fruit_type | 
================================
1   |     ex.jpg   |  apple    | 
2   |     am.png   |  apple    | 
3   |     pl.jpeg  |  orange   |  
4   |      e.png   |  orange   | 
5   |     yep.png  |  apple    |

OK. So when I call this, I want it to pull two pictures, and each should be a random selection from within a fruit_type. So apples should only be picked with apples. Oranges should only be picked with oranges. There should be no combination of both types.

Here's where it breaks down. Let's say the code I run is this:

$query="SELECT * FROM images WHERE fruit_type='apple' 
OR fruit_type='orange' ORDER BY RAND() LIMIT 0,2";

This would return two random selections, like I want. However, this captures BOTH apple and orange types from that column.

I have been trying to identify an SQL statement that would allow it to either choose apples or oranges, but never both. I've come up short, so I'm turning to you all.

Answer 1

您可以创建两个查询，首先是苹果，第二个是橙子，然后选择随机苹果和随机橙色，然后使用UNION ALL子句加入它们。

Answer 2

The following will work, but it won't be a great option from a performance point of view if your table gets huge (millions of records). The below query takes all combinations of images that have the same fruit_type excluding matching images (you don't want the same image twice, do you?).

SELECT images1.*, images2.*
FROM images images1, images images2
WHERE images1.id != images2.id
AND images1.fruit_type = images2.fruit_type
ORDER BY RAND()
LIMIT 0,2

Answer 3

Since you don't have windowing functions in MySQL, you have to improvise. This post is adapted from this blog post by Quassnoi :

  SELECT  
    distinct
    @fi AS file_name,
    fruit_type

    FROM    (
    SELECT  m.*
    FROM    (

    SELECT  @_f = NULL

    ) vars,

    images m
    ORDER BY
    fruit_type, rand()
    ) mo

    WHERE   (CASE WHEN @_f IS NULL OR @_f <> fruit_type THEN @fi := file_name ELSE file_name END IS NOT NULL)

    AND (@_f := fruit_type) IS NOT NULL