php - MySql 连接查询左连接

Question

我有表名 students 具有字段 student_id, house_id 等。和科目，房屋，students_subjects

我正在使用这个查询

SELECT students_subjects.student_id,students.house_id,students_subjects.subject_id,subjects.subject_name,students.rollno,students.first_name, students.last_name FROM 
students_subjects LEFT JOIN students on students_subjects.student_id=students.id
LEFT JOIN subjects on students_subjects.subject_id=subjects.id  WHERE students_subjects.class_years_section_id=1 

这对我来说很好..

现在我也想从 houses 表中获取房屋名称

我试过这个查询

SELECT students_subjects.student_id,students.house_id,houses.house_name, students_subjects.subject_id,subjects.subject_name,students.rollno,students.first_name, students.last_name FROM 
students_subjects LEFT JOIN students on students_subjects.student_id=students.id
LEFT JOIN subjects on students_subjects.subject_id=subjects.id 

LEFT JOIN houses on students.house_id=houses.id WHERE students_subjects.class_years_section_id=1

AND students_subjects.school_session_id=1 AND students.is_active=1

它给了我 house_name = NULL

谁能告诉我如何获得房屋名称。使用连接查询

谢谢

Answer 1

查询中的错误是由 WHERE 子句之后的 LEFT JOIN 关键字引起的，

SELECT  students_subjects.student_id,
        students.house_id,
        students_subjects.subject_id,
        subjects.subject_name,
        students.rollno,
        students.first_name, 
        students.last_name 
FROM    students_subjects 
        LEFT JOIN students 
            on students_subjects.student_id=students.id
        LEFT JOIN subjects 
            on students_subjects.subject_id=subjects.id
        LEFT JOIN houses 
            on students.house_id=houses.id
WHERE   students_subjects.class_years_section_id = 1 AND 
        students_subjects.school_session_id = 1 AND 
        students.is_active = 1

请记住，JOIN 是 FROM 子句的一部分。

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更新 1

SELECT  b.student_id,
        a.house_id,
        b.subject_id,
        c.subject_name,
        a.rollno,
        a.first_name, 
        a.last_name,
        d.house_name            
FROM    students a
        INNER JOIN students_subjects b
            ON b.student_id = a.id
        INNER JOIN subjects  c
            ON b.subject_id = c.id
        INNER JOIN houses d
            ON a.house_id = d.id
WHERE   b.class_years_section_id = 1 AND 
        b.school_session_id = 1 AND 
        a.is_active = 1