MySQL在SELECT中分配var但不显示它

Question

我有这个查询:

SET @id := '';
SELECT 
  libs.song_id,
  songs.name AS song_name,
  artists.id AS artist_id,
  artists.name AS artist_name,
  songs.description,
  libs.status,
  libs.activated,
  libs.giftable,
  @id := libs.song_id,
  IF((SELECT activated FROM libs WHERE @id = song_id && activated = 1), 1, 0) AS duplicate
FROM libs, songs, artists
WHERE
  libs.song_id = songs.id &&
  artists.id = songs.artist_id &&
  libs.activated = 0 &&
  libs.user_id = '1';

一切都按计划进行，但我还得到了一个名为 @id := libs.song_id 的列，我该如何避免它？
谢谢。

Answer 1

首先有几件事。学习使用正确的连接语法。一个简单的规则:只是不要在 from 子句中使用逗号。

其次，您甚至不必将变量初始化为单独的语句。

第三，这个想法很简单。只需在另一个表达式中分配变量，就有一个非常自然的地方可以做到这一点:

SELECT (@id := libs.song_id) as song_id,
       songs.name AS song_name, artists.id AS artist_id, artists.name AS artist_name,
       songs.description, libs.status, libs.activated, libs.giftable,
       IF((SELECT activated FROM libs WHERE @id = song_id AND activated = 1), 1, 0) AS duplicate
FROM libs join
     songs
     on libs.song_id = songs.id join
     artists
     on artists.id = songs.artist_id
WHERE libs.activated = 0 AND libs.user_id = '1';