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SELECT *
FROM `student`
right JOIN `trustee` ON `student`.`trustee` = `trustee`.`id`
right JOIN `bill` ON `student`.`id` = `bill`.`student_id`
where `stage` = 3 &&
`grade` = 3 &&
`month` != null
عندما يتم ازاله الشرط `month` != null يعمل بنجاح واليكم الناتج
SELECT *
FROM `student`
left JOIN `trustee` ON `student`.`trustee` = `trustee`.`id`
left JOIN `bill` ON `student`.`id` = `bill`.`student_id`
where `stage` = 3 &&
`grade` = 3
الناتج Showing rows 0 - 29 ( 159 total, Query took 0.0012 sec)