合并 INNER/LEFT JOIN 中的列返回 NULL 值？

Question

我遇到了一个问题，我连接了多个表，但我需要合并 e.url 和 f.url_new 列。否则，结果如预期。这是我的查询。

SELECT a.`added_date`,b.`type`,c.`action`,e.`url`,f.`url_new` 
FROM `websites_submitted_main` a
INNER JOIN `websites_submitted_type` b ON a.`type_id` = b.`type_id` 
INNER JOIN `websites_submitted_action` c ON a.`action_id` = c.`action_id`
INNER JOIN `users` d ON a.`user_id` = d.`user_id`
LEFT JOIN `websites` e ON a.`url_id` = e.`id`
LEFT JOIN `websites_submitted_new` f ON a.`url_new_id` = f.`url_new_id`
WHERE a.`user_id` = 1 ORDER BY a.`added_date` DESC

我尝试过CONCAT，但该列包含所有NULL 值。这是那个查询。

SELECT a.`added_date`,b.`type`,c.`action`,CONCAT(e.`url`,f.`url_new`) AS url 
FROM `websites_submitted_main` a
INNER JOIN `websites_submitted_type` b ON a.`type_id` = b.`type_id` 
INNER JOIN `websites_submitted_action` c ON a.`action_id` = c.`action_id`
INNER JOIN `users` d ON a.`user_id` = d.`user_id`
LEFT JOIN `websites` e ON a.`url_id` = e.`id`
LEFT JOIN `websites_submitted_new` f ON a.`url_new_id` = f.`url_new_id`
WHERE a.`user_id` = 1 ORDER BY a.`added_date` DESC

我可以对此查询进行细微修改以合并这些列吗？

Answer 1

尝试使用CONCAT_WS() 而不是CONCAT()

SELECT  `CONCAT_WS(' ',e.url, f.url_new )` ....

CONCAT_WS() 将在值不为空时连接这些值。

来自CONCAT()的手册

如果任何参数为 NULL，CONCAT() 将返回 NULL。

SELECT a.`added_date`,b.`type`,c.`action`,CONCAT_WS("", e.`url`,f.`url_new`) AS url 
FROM `websites_submitted_main` a
INNER JOIN `websites_submitted_type` b ON a.`type_id` = b.`type_id` 
INNER JOIN `websites_submitted_action` c ON a.`action_id` = c.`action_id`
INNER JOIN `users` d ON a.`user_id` = d.`user_id`
LEFT JOIN `websites` e ON a.`url_id` = e.`id`
LEFT JOIN `websites_submitted_new` f ON a.`url_new_id` = f.`url_new_id`
WHERE a.`user_id` = 1 ORDER BY a.`added_date` DESC