PHP和MySQL子查询，其中id位于（）中

我正在尝试运行此查询：

  $sql = "SELECT p.*, c.id as cid, c.name as catname, p.title as ptitle,"
  . "\n CONCAT(u.first,' ',u.last) as user," 
  . "\n (SELECT GROUP_CONCAT(DISTINCT tags) as tagnames FROM tagtable WHERE id IN (p.tagids))" 
  . "\n FROM posts as p" 
  . "\n LEFT JOIN categories as c ON c.id = a.cid" 
  . "\n LEFT JOIN users as u ON u.id = a.uid" 
  . "\n WHERE a.active = 1 LIMIT 1";

我唯一的问题是子查询

“选择组（不同的标记）…”

出于某种原因，它不会产生任何结果。我希望得到标记名行作为

“lorem、ipsum、dolores，…”

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字段标记ID采用

1,2,3

格式。

如果标记ID采用“1,2,3”格式，则应使用

如果字符串str位于 由N个子字符串组成的字符串列表strlist

比如

WHERE FIND_IN_SET(id,p.tagids) > 0

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你是如何得到p.tagids是1,2,3格式的？