Question

New to MySQL, so please bear with me.

I’m working on a project that collects user’s degrees. Users can save 3 degrees where the type, subject matter, and school are variable. These relations are normalized for other query uses so 5 tables are involved and are shown below (all have more columns then shown, just included the relevant info). The last one, ‘user_degrees’ is where the keys come together.

degrees

+----+-------------------+
| id | degree_type       |
+----+-------------------+
| 01 | Bachelor's Degree |
| 02 | Master's Degree   |
| 03 | Ph.D.             |
| 04 | J.D.              |
+----+-------------------+

acad_category

+------+-----------------------------------------+
| id   | acad_cat_name                           | 
+------+-----------------------------------------+
| 0015 | Accounting                              |
| 0026 | Business Law                            |
| 0027 | Finance                                 |
| 0028 | Hotel & Restaurant Management           |
| 0029 | Human Resources                         |
| 0030 | Information Systems and Technology      |
+------+-----------------------------------------+

institutions

+--------+--------------------------------------------+
| id     | inst_name                                  |
+--------+--------------------------------------------+
| 000001 | A T Still University of Health Sciences    |
| 000002 | Abilene Christian University               |
| 000003 | Abraham Baldwin Agricultural College       |
+------+----------------------------------------------+

users

+----------+----------+
| id       | username |
+----------+----------+
| 00000013 | Test1    |
| 00000018 | Test2    |
| 00000023 | Test3    |
+----------+----------+

user_degrees

+---------+-----------+---------+---------+
| user_id | degree_id | acad_id | inst_id |
+---------+-----------+---------+---------+
|      18 |         1 |       4 |       1 |
|      23 |         1 |      15 |       1 |
|      23 |         2 |      15 |       1 |
|      23 |         3 |      15 |       1 |
+---------+-----------+---------+---------+

How can I query ‘user_degrees’ to find all degrees by user x, but return the actual values of the foreign keys? Taking user Test3 as an example, I’m looking for output like so (truncated for layout’s sake):

+-------------------+-------------------+-------------------+
| degree_type       | acad_cat_name     | inst_name         |
+-------------------+-------------------+-------------------+
| Bachelor's Degree | Accounting        | A T Still Uni..   |
| Master's Degree   | Accounting        | A T Still Uni..   |            
| Ph.D.             | Accounting        | A T Still Uni..   |
+-------------------+-------------------+-------------------+

I’m guessing a mix of multiple joins, temp tables and subqueries are the answer but am having trouble grasping the order of things. Any insight is much appreciated, thanks for reading.

Answer 1

You need to join user_degrees to degrees (and the other tables referenced by user_degrees). This is the query that will give you your example output:

SELECT
  ud.user_id, d.degree_type, ac.acad_cat_name, i.inst_name
FROM
 user_degrees ud
 INNER JOIN degrees d ON d.id = ud.degree_id
 INNER JOIN acad_category ac ON ac.id = ud.acad_id
 INNER JOIN institutions i ON i.id = ud.inst_id
WHERE
 ud.user_id = 18

You may also want to read this article to understand different kinds of joins: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

The only way to understand these things at your stage of learning is to actually write the queries and then modify them until you get your desired output.