mysql - SQL JOIN 变量不起作用

Question

我正在尝试使用 2 个左连接从 3 个表中选择变量。

SELECT a.username, a.id, COUNT(c.featured)
FROM user a
LEFT JOIN board b
ON a.id = b.user_id
LEFT JOIN pins c
ON a.id = c.user_id
WHERE c.featured='yes'
GROUP BY b.board_name
ORDER BY COUNT(c.featured) ASC
LIMIT 3

这应该从数据库中获取 3 个得分最高的板 block ，并根据它们出现的次数升序排列。我用它来回显:

".$info['b.board_name']."

但是没有显示任何内容

Answer 1

我不确定您是否想要 b.boardname 或 a.username, a.id 在您的 SELECT 中，但您有相应地执行GROUP BY:

尝试这样做:

SELECT a.username, a.id, COUNT(c.featured)
FROM user a
LEFT JOIN board b ON a.id = b.user_id
LEFT JOIN pins c ON a.id = c.user_id
WHERE c.featured = 'yes'
GROUP BY a.username, a.id
ORDER BY COUNT(c.featured) DESC LIMIT 3

或者

SELECT b.board_name, COUNT(c.featured)
FROM user a
LEFT JOIN board b ON a.id = b.user_id
LEFT JOIN pins c ON a.id = c.user_id
WHERE c.featured = 'yes'
GROUP BY b.board_name
ORDER BY COUNT(c.featured) DESC LIMIT 3