Question

What is the best way to sort a result set in MySQL based on a field value that could come from multiple tables?

For the sake of simplicity, assume there are four tables in the DB. The primary table contains a number of fields, but particularly it contains a column with the name of another table and a foreign key to a record in said table. Each of the three foreign tables contain a name column that I would like to sort the results by in the primary table.

Example primary table:

+----+---------------+------------+-------------+
| id | foreign_table | foreign_id | more_fields |
+----+---------------+------------+-------------+
|  1 | students      |       9182 | blah, blah  |
|  2 | students      |       1008 | blah, blah  |
|  3 | parents       |       3827 | blah, blah  |
|  4 | teachers      |       4523 | blah, blah  | 
|  5 | teachers      |       1092 | blah, blah  | 
+----+---------------+------------+-------------+

Example foreign (students) table:

+-------+--------------+-------------+
| id    | name         | more_fields |
+-------+--------------+-------------+
|  9182 | Joe          |  blah, blah | 
|  1008 | Sally        |  blah, blah | 
+-------+--------------+-------------+

Is this possible in MySQL? Or do I need to loop through the results in PHP and create an array to sort by and then use something like array_multisort()? If using an array, would it be efficient to loop through thousands of rows just to sort the results?

Any help is greatly appreciated.

Answer 1

Using three LEFT JOINs (students, parents, teachers) and listing each in the ORDER BY ought to do the job if you COALESCE() the names from the other three tables together:

SELECT
  primary_table.*,
  COALESCE(students.name, parents.name, teachers.name) AS name
FROM 
  primary_table
  LEFT JOIN students ON (foreign_table = 'students' AND foreign_id = students.id)
  LEFT JOIN parents ON (foreign_table = 'parents' AND foreign_id = parents.id)
  LEFT JOIN teachers ON (foreign_table = 'teachers' AND foreign_id = teachers.id)
ORDER BY name 

In the long run, I would suggest changing this schema to store all of the different types of names in one table, with an identifier in that table indicating whether it is a student, teacher, or parent.