Question

如何找到2个用户之间的公共列，但如果没有，则返回零结果，这意味着即使没有公共列也会返回结果。

SELECT u.usr_id, u.usr_fname, u.usr_lname, COUNT(ul.ulnk_lnk_id) AS common_links
FROM users AS u
LEFT JOIN user_links AS ul ON ul.ulnk_usr_id=u.usr_id
WHERE u.usr_id != $user_id
AND ul.ulnk_lnk_id IN
(SELECT l.ulnk_lnk_id FROM user_links AS l WHERE l.ulnk_usr_id=$user_id) 
GROUP BY u.usr_id HAVING common_links >= 0 ORDER BY common_links DESC";

这将返回用户之间有一个或多个公共列的结果，我需要返回一个包含0个常用列的结果......

Answer 1

您的where子句排除了没有共同链接的任何用户。将ul.ulnk_lnk_id检查移至ON子句应该可以解决问题。

SELECT u.usr_id, u.usr_fname, u.usr_lname, COUNT(ul.ulnk_lnk_id) AS common_links
FROM users AS u
LEFT JOIN user_links AS ul ON (ul.ulnk_usr_id=u.usr_id AND ul.ulnk_lnk_id IN
(SELECT l.ulnk_lnk_id FROM user_links AS l WHERE l.ulnk_usr_id=$user_id) )
WHERE u.usr_id != $user_id
GROUP BY u.usr_id 
ORDER BY common_links DESC

Answer 2

试试这个：

SELECT u.usr_id, u.usr_fname, u.usr_lname, COUNT(ul.ulnk_lnk_id) AS common_links
FROM users AS u
LEFT JOIN user_links AS ul ON ul.ulnk_usr_id=u.usr_id
WHERE u.usr_id != $user_id
AND ul.ulnk_lnk_id IN
(SELECT l.ulnk_lnk_id FROM user_links AS l WHERE l.ulnk_usr_id=$user_id) 
GROUP BY u.usr_id  ORDER BY common_links DESC";

查找两个用户之间的公共列，如果没有，则返回null /零

2 个答案: