php - 限制随机返回的 id，但每个 id 的行数未知

Question

我想从我的数据库中随机选择 5 个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:我不想限制返回的行，我只想限制不同的 customer_id。 但是因为我必须随机选择它们，所以我不能使用数学(例如“WHERE customer_id < 6”)。 有没有办法随机选择 5 个客户并在同一查询中返回他们的所有 food_preferences？

Answer 1

首先，永远不要在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 放入子查询中即可:

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;

如果并非所有客户都在 listing 中，而您只希望客户在 listing 中，那么您可以添加另一个加入:

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