Question

我有3个表：制造商，产品和客户我有一个查询，让我所有的制造商与他们的产品和他们的客户。我用两个LEFT JOINS做到了这一点。

对于10个制造商而言，结果超过500行，因为对于产品和客户表中的每个条目，它们是结果中具有制造商ID的行。

我想创建一个查询：向我的制造商提供他们的产品和他们的客户，但不是所有的制造商，只有10或20或有限的数量。

我该怎么做？我在想这个......

SELECT * 
FROM manufacturer as m
LEFT JOIN product as p ON ( m.mn_id = p.man_mn_id )
LEFT JOIN client  as c ON ( m.mn_id = c.man_mn_id )
WHERE m.mn_id IN ( SELECT mn_id
                   FROM manufacturer
                   WHERE mn_id >= ( SELECT FLOOR( MAX(mn_id) * RAND() )
                                    FROM manufacturer
                                  )
                   ORDER BY mn_id
                   LIMIT 10
                 )

或者他们是更好的方式？

Answer 1

您可以通过限制制造商来尝试这一点，它将获得10个制造商，并将这些制造商加入他们的产品和客户

SELECT 
  * 
FROM
  (SELECT 
    * 
  FROM
    manufacturer 
  ORDER BY id DESC 
  LIMIT 10) AS m  /* you can change limit to 20 or something you want */
  LEFT JOIN product AS p 
    ON (m.mn_id = p.man_mn_id) 
  LEFT JOIN CLIENT AS c 
    ON (m.mn_id = c.man_mn_id)

Answer 2

我认为您可以将查询简化为类似的内容，保留查询中的“随机”功能：

SELECT * 
FROM (SELECT * FROM manufacturer ORDER BY RAND() LIMIT 10) as m
LEFT JOIN product as p ON (m.mn_id = p.man_mn_id)
LEFT JOIN client as c ON (m.mn_id = c.man_mn_id)
ORDER BY mn_id

如何限制唯一ID的出现次数

2 个答案: