Jquery where子句中的第一个子查询返回？

以下音乐库数据库：

以及以下查询：

SELECT CDNumber

FROM CD

WHERE CDNumber NOT IN

(SELECT T2.CDNumber

 FROM Track AS T1 JOIN Singer AS S1 ON T1.PerformanceCode = S1.PerformanceCode

 JOIN Singer AS S2 ON T1.PerformanceCode = S2.PerformanceCode

 WHERE S1.SingerName <> S2.SingerName)

 AND CDNumber IN

 (SELECT T2.CDNumber

  FROM Track AS T2 JOIN Recording AS R ON T2.PerformanceCode = R.Performance

  WHERE R.Year < CD.Year

  GROUP BY CDNumber

  HAVING count(*) >= 3)

问题：where子句中的第一个子查询是否返回所有歌曲都由单个歌手演唱的CD数量？是/否，我如何知道这一点？

是，因为

    WHERE S1.SingerName <> S2.SingerName

单曲歌手永远不会有两个不同的歌手名字