限制随机返回的 ID，但每个 ID 的行数未知

Question

我想 select 从我的数据库中随机抽取 5 位用户并显示他们所有的食物偏好。

目前我有这些表：

CUSTOMERS
customer_id email 

FOOD_PREFERENCE
food_id food_name allergic_info

LISTING
customer_id food_id

我的查询必须与此类似：

SELECT c.email, f.food_name, f.allergic_info
FROM customers c, food_preference f, listing l
WHERE l.customer_id=c.customer_id AND f.food_id=l.food_id
ORDER BY rand(c.customer_id) LIMIT 10

问题是：我不想限制 returned 的行，我只想限制不同的 customer_id。但是因为我必须随机 select 它们，所以我不能使用数学（例如 "WHERE customer_id < 6"）。有没有办法在同一查询中随机 select 5 位客户和 return 他们所有的 food_preferences？

Answer 1

您可以在子查询中限制客户，然后然后执行 JOIN。

此外，建议使用现代显式 JOIN 语法。

select c.email,
    f.food_name,
    f.allergic_info
from listing l
join (
    select *
    from customers
    order by rand() limit 10
    ) c on l.customer_id = c.customer_id
join food_preference f on f.food_id = l.food_id;

Answer 2

首先，从不在FROM子句中使用逗号。始终使用显式 JOIN 语法。

所以，您的查询应该是：

SELECT c.email, f.food_name, f.allergic_info
FROM listing l JOIN
     customers c  
     ON l.customer_id = c.customer_id JOIN
     food_preference f
     ON f.food_id = l.food_id
ORDER BY rand(c.customer_id)  -- I don't know why you are providing a see here
LIMIT 10;

如果所有客户都有食物偏好，只需将 limit 放入子查询中：

SELECT c.email, f.food_name, f.allergic_info
FROM listing l JOIN
     (SELECT c.*
      FROM customers c  
      ORDER BY rand()
      LIMIT 5
     ) c
     ON l.customer_id = c.customer_id JOIN
     food_preference f
     ON f.food_id = l.food_id;

如果不是所有客户都在 listing 而您只想要 listing 的客户，那么您可以添加另一个 join:

SELECT c.email, f.food_name, f.allergic_info
FROM listing l JOIN
     customers c
     ON l.customer_id = c.customer_id JOIN
     food_preference f
     ON f.food_id = l.food_id JOIN
     (SELECT customer_id
      FROM (SELECT DISTINCT customer_id FROM LISTING) lc
      ORDER BY rand()
      LIMIT 5
     ) lc
     ON l.customer_id = lc.customer_id

限制随机返回的 ID，但每个 ID 的行数未知

Limit random returned id's, but with unknown row count per id

php

mysql

random

row

limit