Returner NULL, hvis Count(*) er nul

For det første mangler du en GROUP BY klausul nederst i din forespørgsel for at gruppere efter school_name :

SELECT count(student_name) AS total_student, school_name
FROM student
    LEFT JOIN school_info ON school_info.school_id = student.school_id
WHERE student.status = '0'
GROUP BY school_name

Så, hvis du simpelthen ikke vil vise rækker, hvor total_student =0, kan du bruge MySQL HAVING-sætningen:

SELECT count(student_name) AS total_student, school_name
FROM student
    LEFT JOIN school_info ON school_info.school_id = student.school_id
WHERE student.status = '0'
GROUP BY school_name
HAVING count(student_name) > 0

Eller du kan ændre LEFT JOIN til INNER JOIN at opnå det samme i dette tilfælde.

Endelig, hvis du i stedet vil erstatte 0 med null, men stadig har rækker, kan du opdatere select-sætningen og få totalerne til:

SELECT IF(COUNT(student_name) = 0, NULL, COUNT(student_name)) AS total_student, school_name