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

但我想要的是在没有教师的科目中给予“N / A”。实施例

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

我将如何在mySQL中设置条件，以便结果看起来完全符合上述示例？

提前致谢

Answer 1

LEFT JOIN代替INNER JOIN，如此：

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。 ON子句后面没有JOIN子句。您必须将连接条件放在ON子句之后。

Answer 2

试试这个::

使用左连接和ifnull

SELECT subjects.name as name , grades.grade as grade,  ifnull(concat(instructors.lastname, ", ", instructors.firstname , " ", left(instructors.middlename, 1),"."),'N/A)' as instructor
    FROM subjects
    LEFT JOIN grades on subjects.id = grades.subject_id
    LEFT JOIN instructors on instructors.subject_id = subjects.id
    WHERE grades.student_id = 1

Answer 3

您可以Left join或right join代替join或inner join。如果没有相关数据，Left Join将显示左侧的所有数据，右侧显示null。所以你的查询将成为：

SELECT subjects.name as name , grades.grade as grade,  
isnull(concat(instructors.lastname, ", ", instructors.firstname , " ",
left(instructors.middlename, 1),"."), 'N/A') as instructor
FROM subjects
LEFT JOIN grades ON subjects.id = grades.subject_id 
LEFT JOIN instructors  ON instructors.subject_id = subjects.id
WHERE grades.student_id = 1

如果mysql中不存在，请设置另一个值

3 个答案:

SQL Fiddle Demo