sql - 基于第一个响应的第二个表的 SQL 查询

Question

这是一个 SQL 查询，如果 first- 返回多个值，则该查询失败

SELECT     subject
FROM         a_batchcourse_subject AS s
WHERE     (batchcourse =
                          (SELECT     batchcourse
                          FROM          a_course_batchcourse AS b
                          WHERE      (course = 'Course 1')))

在第一个查询可能返回多个值的情况下，编写此查询的正确方法是什么？

Answer 1

你不想IN在 sql 中=

SELECT subject
FROM a_batchcourse_subject AS s
WHERE batchcourse IN
(SELECT batchcourse
FROM a_course_batchcourse AS b
WHERE course = 'Course 1')

Answer 2

JOIN比IN从句快得多

SELECT  s.subject
FROM    a_batchcourse_subject s 
        INNER JOIN a_course_batchcourse b
        on s.batchcourse = b.batchcourse
WHERE   (b.course = 'Course 1')