如何返回外键存在信息来选择结果？

Question

我有一个问题：

    SELECT `Name`, `ID_dir`, 999 as `children` 
    FROM `dir` dir WHERE dir.`fid_parent` IS NULL 
    AND (
      EXISTS (
         SELECT 1
         FROM   `file` f
         WHERE  dir.ID_dir = f.fid_parent
         )
       OR (
         SELECT 1
         FROM   `dir` d2
         WHERE  dir.ID_dir = d2.fid_parent
       )
    )

我在哪里检查目录是否有任何外键。 如何在“Select ... 999 as children”中移动该信息代替 999？ 我想在那个地方返回 (0 或 1) xor Boolean 作为children。

Answer 1

将EXISTS 子查询放入SELECT 列表中。

SELECT Name, ID_Dir, (
      EXISTS (
         SELECT 1
         FROM   `file` f
         WHERE  dir.ID_dir = f.fid_parent
         )
       OR (
         SELECT 1
         FROM   `dir` d2
         WHERE  dir.ID_dir = d2.fid_parent
       )
    ) AS children
FROM dir WHERE fid_parent IS NULL