Mysql- SELECT列'A',即使使用NULLS
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表A包含学生姓名,表B和C包含班级和学生的存在。我想展示所有学生并参加他们的出席。问题是我无法显示所有没有检查过的学生。在我检查学生在场的情况下,这是可以的,但如果在给定的课程中,在给定的一天和给定的科目中没有检查过的存在 - 则不显示任何内容。
我的查询:
SELECT student.id_student, CONCAT(student.name,' ' ,student.surname) as 'name_surname',pres_student_present, pres_student_absent, pres_student_justified, pres_student_late, pres_student_rel, pres_student_course, pres_student_delegation, pres_student_note FROM student
LEFT JOIN class ON student.no_classes = class.no_classes
LEFT JOIN pres_student ON student.id_student = pres_student.id_student
WHERE (class.no_classes = '$class' OR NULL AND pres_student_data = '$data' AND pres_student_id_subject = $id_subject OR NULL)
GROUP BY student.surname
ORDER BY student.surname ASC
我想始终显示name_surname,任何其他列应该有NULL或1像:
Name | present | absent | just | late | rel | delegation | note |
Donald Trump | 1 | | | | | | |
Bush | | | | | | | |
Someone | 1 | | | | | | |
等...
您应该将class
和pres_student
tables的限制从WHERE
子句移到ON
(LEFT join)。
在你使用WHERE
对outer join
子句执行限制的情况下,sql引擎认为你正在执行INNER join
SELECT student.id_student
, CONCAT(student.name, ' ', student.surname) AS 'name_surname'
, pres_student_present
, pres_student_absent
, pres_student_justified
, pres_student_late
, pres_student_rel
, pres_student_course
, pres_student_delegation
, pres_student_note
FROM student
LEFT JOIN class
ON student.no_classes = class.no_classes
AND class.no_classes = '$class'
LEFT JOIN pres_student
ON student.id_student = pres_student.id_student
AND pres_student_data = '$data'
AND pres_student_id_subject = $id_subject
GROUP BY student.surname
ORDER BY student.surname ASC
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