mysql - 如果 mysql 中不存在则设置另一个值

Question

SELECT subjects.name as name , grades.grade as grade,  concat(instructors.lastname, ", ", instructors.firstname , " ", left(instructors.middlename, 1),".") as instructor
    FROM subjects
    JOIN grades
    JOIN instructors
    WHERE grades.student_id = 1 AND subjects.id = grades.subject_id AND instructors.subject_id = subjects.id

结果是

English    Instructor    Grade
Math       Instructor    Grade

但是我想要的是在还没有导师的科目中给出“N/A”。示例

English    Instructor    Grade
Math       Instructor    Grade
Science    N/A           Grade
History    N/A           Grade

如何在 mySQL 中设置条件，以便结果与上面的示例完全相同？

提前致谢

Answer 1

LEFT JOIN 而不是 INNER JOIN，如下所示:

SELECT 
  s.name as name , 
  g.grade as grade,  
  IFNULL(concat(i.lastname, ", ", i.firstname, ...), 'N/A') as instructor
FROM subjects s
LEFT JOIN grades g ON g.subject_id = s.id 
LEFT JOIN instructors i ON g.subject_id = i.subject_id ;

SQL Fiddle Demo

请注意:您编写JOIN的方式不正确。 JOIN 子句之后没有 ON 子句。您必须将连接条件放在 ON 子句之后。