好的，这里有一个现有的查询，可以满足我的需求。
SELECT C1.id AS id, C1.title AS title, C1.instructor_id AS instructor_id,
C1.description AS description, C1.date AS date, C1.starttime AS starttime,
C1.endtime AS endtime, C1.training_id AS training_id, C2.name AS name,
C2.id AS instructors_id
FROM trainings C1
LEFT JOIN instructors C2 ON C1.instructor_id = C2.id
然而，我需要补充一些东西。除了我现有的表之外，我还有两张表需要比较。
表"trainings"有一个培训课程列表，按"id"进行索引。我还有另一个表"registrations"，它包含关于哪些用户注册了哪些培训课程的信息，共有3列："id"-索引，"user_id"-注册培训的用户的id，"course_id"–培训课程的id。
除了我现有的查询之外，我需要做的是只选择"注册"中没有该用户行的培训课程。
我希望这是有道理的。谢谢你的帮助。我已经试了好几个小时了。这个查询太大了，我无法保持组织并正确思考如何进行。
SELECT C1.id AS id, C1.title AS title, C1.instructor_id AS instructor_id,
C1.description AS description, C1.date AS date, C1.starttime AS starttime,
C1.endtime AS endtime, C1.training_id AS training_id, C2.name AS name,
C2.id AS instructors_id
FROM trainings C1
LEFT JOIN instructors C2 ON C1.instructor_id = C2.id
LEFT JOIN registrations C3 ON < Whatever they're connected on >
WHERE C3.id IS NULL
基本上也只使用JOIN，并通过WHERE ... IS NULL进行过滤，因为LEFT JOIN将以NULL的形式返回联接表的列。