Question

Here is a SQL query which fail if multiple values are returned by first-

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

what is the right way to write this query where first query may return multiple values ?

Answer 1

JOIN is much faster than IN clause

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