How can I left join multiple data from the same mySQL table? - php

Question

This is my table position:
+----+---------+------+--------+---------+
| id | teacher | cook | doctor | dentist |
+----+---------+------+--------+---------+
| 1 | 3 | 4 | 2 | 1 |
+----+---------+------+--------+---------+
And this is my table people:
+----+-----------+--------+-----+
| id | firstname | name | age |
+----+-----------+--------+-----+
| 1 | Fred | Miller | 42 |
| 2 | Emily | Rose | 32 |
| 3 | Ben | Harper | 38 |
| 4 | Samanta | Jones | 35 |
+----+-----------+--------+-----+
My request from the mySQL database
$pdo = $db->query('
SELECT *, position.id AS id, people.id AS people_id
FROM position
LEFT JOIN people
ON position.teacher=people.id;
ON position.cook=people.id;
ON position.doctor=people.id;
ON position.dentist=people.id;
');
while ($row = $pdo->fetch(PDO::FETCH_ASSOC)) {
echo "The teacher is "$row['firstname']." ".$row['name'];
echo "The cook is "$row['firstname']." ".$row['name'];
echo "The doctor is "$row['firstname']." ".$row['name'];
echo "The dentist is "$row['firstname']." ".$row['name'];
}
My result is:
The teacher is Ben Harper
The cook is Ben Harper
The doctor is Ben Harper
The dentist is Ben Harper
The result I need:
The teacher is Ben Harper
The cook is Samanta Jones
The doctor is Emiliy Rose
The dentist is Fred Miller

Answer 1

It will be better if you restructure your table.
+----+------------+
| id | position |
+----+------------|
| 1 | teacher |
| 2 | cook |
| 3 | doctor |
| 4 | dentist |
+----+------------+
+----+-----------+--------+-----+-------------+
| id | firstname | name | age | position_id |
+----+-----------+--------+-----+-------------+
| 1 | Fred | Miller | 42 | 4 |
| 2 | Emily | Rose | 32 | 3 |
| 3 | Ben | Harper | 38 | 1 |
| 4 | Samanta | Jones | 35 | 2 |
+----+-----------+--------+-----+-------------+
Here you can have a foreign key reference on people.position_id and position.id.
This way you can have many people with same position.
SELECT position.id id
, people.id people_id
, people.firstname
, people.name
FROM position
LEFT
JOIN people
ON people.position_id = position.id;

Answer 2

As noted in comments and answers above, if you can restructure your table, you absolutely should -- as others have mentioned, it's better to restructure it to have that extra column, and then your query will be both simpler and much more efficient. But if you're stuck in a position where you can't (and if you only have a SMALL amount of data as presented, so that the inefficiency of this isn't too much of a concern), then you could use a UNION to glue several simple queries together :
Select position.id, firstname, lastname, 'cook' AS position
FROM position LEFT JOIN people ON position.cook = people.id
UNION
Select position.id, firstname, lastname, 'teacher' AS position
FROM position LEFT JOIN people ON position.teacher = people.id
UNION
...
Again, this is NOT an efficient approach -- you should restructure your data if you can. But if you can't for whatever reason, then this would get you back the data you need in a set of rows that pulls back what you need.

Answer 3

Try this,
$pdo = $db->query('
SELECT *, position.id AS id, people.id AS people_id,
(case when position.teacher <> '' then 'teacher'
when position.cook <> '' then 'cook'
when position.doctor <> '' then 'doctor'
when position.dentist <> '' then 'dentist') position_name
FROM position
LEFT JOIN people
ON position.teacher=people.id;
ON position.cook=people.id;
ON position.doctor=people.id;
ON position.dentist=people.id;
');
while ($row = $pdo->fetch(PDO::FETCH_ASSOC)) {
echo "The ".$row['position_name']." is "$row['firstname']." ".$row['name'];
}