Join only for specific rows where value matches a variable - mysql

Question

I have multiple MySQL tables containing varying numbers of columns. After joining three of the tables, I have a resulting table that's structured as follows:
+------------+------------+-----------+-------+------+
| student_id | first_name | last_name | class | rank |
+------------+------------+-----------+-------+------+
| 1 | John | Doe | 2012 | 1 |
+------------+------------+-----------+-------+------+
| 2 | Suzy | Public | 2013 | 12 |
+------------+------------+-----------+-------+------+
| 3 | Mike | Smith | 2014 | 50 |
+------------+------------+-----------+-------+------+
I also have two additional tables that aren't involved in the initial join:
interest
+-------------+------------+-----------------------+----------------+
| interest_id | student_id | employer_interest | interest_level |
+-------------+------------+-----------------------+----------------+
| 1 | 1 | Wayne Enterprises | High |
+-------------+------------+-----------------------+----------------+
| 2 | 1 | Gotham National Bank | Medium |
+-------------+------------+-----------------------+----------------+
| 3 | 2 | Wayne Enterprises | Low |
+-------------+------------+-----------------------+----------------+
| 4 | 3 | Gotham National Bank | High |
+-------------+------------+-----------------------+----------------+
offers
+----------+------------+-----------------------+
| offer_id | student_id | employer_offer |
+----------+------------+-----------------------+
| 1 | 1 | Wayne Enterprises |
+----------+------------+-----------------------+
| 2 | 1 | Gotham National Bank |
+----------+------------+-----------------------+
| 3 | 2 | Wayne Enterprises |
+----------+------------+-----------------------+
The interest and offers table won't necessarily contain a record for every student_id but at the same time contain multiple records that reference a single student_id.
For each of the latter two tables, I'd like to:
Select all rows where the employer_interest or employer_offer value is equal to $var (a variable I've set in PHP)
Join these rows to the original table
For example, if $var is set to Wayne Enterprises, I'd like the resulting table to be:
+------------+------------+-----------+-------+------+-------------------+----------------+-------------------+
| student_id | first_name | last_name | class | rank | employer_interest | interest_level | employer_offer |
+------------+------------+-----------+-------+------+-------------------+----------------+-------------------+
| 1 | John | Doe | 2012 | 1 | Wayne Enterprises | High | Wayne Enterprises |
+------------+------------+-----------+-------+------+-------------------+----------------+-------------------+
| 2 | Suzy | Public | 2013 | 12 | Wayne Enterprises | Low | Wayne Enterprises |
+------------+------------+-----------+-------+------+-------------------+----------------+-------------------+
| 3 | Mike | Smith | 2014 | 50 | NULL | NULL | NULL |
+------------+------------+-----------+-------+------+-------------------+----------------+-------------------+
Is what I'm trying to do possible using just a MySQL query? If so, how do I do it?

Answer 1

it sounds like you just need a LEFT JOIN to the other tables since it appears you want to see all students from the first set regardless of any job offer/interest.
If so... ensure both the "Interest" and "Offers" tables have an index where the student ID is either a single element index, or first in that of a compound index.
select STRAIGHT_JOIN
ORS.Student_ID,
ORS.First_Name,
ORS.Last_Name,
ORS.Class,
ORS.Rank,
JI.Employer_Interest,
JI.Interest,
OFR.Employer_Offer
from
OriginalResultSet ORS
LEFT JOIN Interest JI
ON ORS.Student_ID = JI.Student_ID
AND JI.Employer_Interest = YourPHPVariable
LEFT JOIN Offers OFR
on JI.Student_ID = OFR.Student_ID
AND JI.Employer_Interest = OFR.Employer_Offer
To prevent "NULL" results in the employer interest, interest and offer, you can wrap them in a Coalesce() call such as (for all three columns on left join)
COALESCE( JI.Employer_Interest, " " ) Employer_Interest

Answer 2

Your query should be something like this:
select
s.student_id, s.first_name, s.last_name, s.class, s.rank,
i.employer_interest, i.interest_level,
o.employer_offer
from students s
left join interest i
on i.student_id = s.student_id
and i.employer_interest = 'Wayne Enterprises'
left join offers o
on o.student_id = s.student_id
and o.employer_offer = 'Wayne Enterprises'