SQL Missing Marks Rule | Dataford Interview Questions

PayPal's analyst training dataset contains one row per student and subject. Before calculating performance metrics, exclude students whose marks are missing in two or more subjects.

Write a PostgreSQL query that returns the students who should remain after this exclusion.

Requirements

Count missing marks separately for each student, where a missing mark is represented by NULL.
Exclude students with missing marks in two or more subjects.
Return each retained student's ID and name, ordered by student_id.

Each student has at most one row per subject, so each NULL mark represents one missing subject mark.

Schema

student_marks

Column	Type	Description
student_id	INT	Student identifier
student_name	VARCHAR(100)	Student's full name
subject_name	VARCHAR(100)	Subject being assessed
marks	NUMERIC(5,2)	Marks earned, or NULL when the mark is missing

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