限制随机返回的ID，但每个ID的行数未知

Question

我想从数据库中选择5个随机用户，并显示他们所有的食物偏爱。

目前，我有这些表：

CUSTOMERS
customer_id email 

FOOD_PREFERENCE
food_id food_name allergic_info

LISTING
customer_id food_id

我的查询必须类似于以下内容：

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，由于我必须随机选择它们，因此无法使用数学运算（例如“ WHERE customer_id <6”）。 有没有一种方法可以随机选择5个客户，并在同一查询中返回他们所有的food_preferences？

Answer 1

首先， 切勿在FROM子句中使用逗号。 始终使用显式的JOIN语法。

因此，您的查询应为：

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爱，只需将limit输入子查询即可：

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;

如果不是所有客户都在listing而您只希望客户在listing ，则可以添加另一个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

Answer 2

您可以在子查询中限制客户， 然后进行联接。

另外，建议使用现代的显式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;