根据另一个表中的行类型选择行

Question

This is my database structure.

Member Table(Type - S: Student, T: Teacher)

+---+----+----+----+
|idx|type| id | pw |
+---+----+----+----+
| 1 | S  | A  | .. |
| 2 | S  | B  | .. |
| 3 | T  | C  | .. |
| 4 | T  | D  | .. |
| 5 | S  | E  | .. |
| 6 | S  | F  | .. |
+---+----+----+----+

Student Table

+---+-----+-----+------+
|idx|grade|class|number|
+---+-----+-----+------+
| 3 |   3 |   8 |   29 |
| 4 |   2 |  10 |   13 |
+---+-----+-----+------+

Teacher Table

+---+-------+
|idx|enabled|
+---+-------+
| 3 | N     |
| 4 | N     |
+---+-------+

I want to get member info with specific info according to member type.

my code is

$result = query("SELECT * FROM `member` WHERE `id` = '...' AND `pw` = '...'");
$member_info = fetch_obj($result);

if ($member_info->type === 'T') {
    $result = query("SELECT * FROM `teacher` WHERE `idx` = '...'");
}
else {
    $result = query("SELECT * FROM `student` WHERE `idx` = '...'");
}
$specific_info = fetch_obj($result);

But, I want to get all data in just one query request.
like:

SET @type = SELECT `type` FROM `member` WHERE `member`.`idx` = '2';

SELECT 
    CASE @type
    WHEN 'S' THEN `member`.*, `student`.*
    WHEN 'T' THEN `member`.*, `teacher`.*
    END
FROM 
    CASE @type
    WHEN 'S' THEN `member`, `student`
    WHEN 'T' THEN `member`, `teacher`
    END
WHERE
    `member`.`idx` = '2' AND
    CASE @type
    WHEN 'S' THEN
        `student`.`idx` = `member`.`idx`
    WHEN 'T' THEN
        `teacher`.`idx` = `member`.`idx`
    END
;

How can I do?

Answer 1

If the student and teacher tables have the same columns then the easiest solution is probably to UNION 2 queries together, working on the basis that one of the unioned queries will not return anything.

Something like this, although I would specify the columns I wanted returned and not use SELECT *

SELECT `member`.*, `student`.*
FROM `member`
INNER JOIN `student`
ON `student`.`idx` = `member`.`idx`
WHERE `member`.`idx` = '2' 
AND member.type = 'S'
UNION
SELECT `member`.*, `teacher`.*
FROM `member`
INNER JOIN `teacher`
ON `teacher`.`idx` = `member`.`idx`
WHERE `member`.`idx` = '2' 
AND member.type = 'T'

Answer 2

Since it appears in your data the Idx column in your member table could represent both that of a student AND a teacher, I would do a double left-join. Something like (and I try to never use select * )

select
      m.idx,
      m.type,
      m.id,
      m.pw,
      case when s.idx IS NULL then 0 else 1 end as IsStudent,
      s.grade,
      s.class,
      s.number,
      case when t.idx IS NULL then 0 else 1 end as IsTeacher,
      t.enabled
   from
      Member m
         LEFT JOIN Student s
            on m.idx = s.idx
         LEFT JOIN Teacher t
            on m.idx = t.idx
   where
      m.idx = 2

Then, in your result set, you will have two "flag" (0 or 1) columns IsStudent and IsTeacher so you could conditionally use the other columns IF they are so available.