SQL find rows in the same table based on certain columns - mysql

Question

I have two tables which are for two different programs. Each program has a specific program_instance (the program_instance) is the year of the program.
One table is called 'enrollees' and the other is 'nominations' - for two programs that aren't technically related.
I've been trying to get the count of past participants from both tables. For reference:
program_instance_id:
5 = GC 2014
3 = GC 2013
1 = GC 2012
4 = GE 2013
2 = GE 2012
So I ran this query on my enrollees table and it produced a result in 913ms:
SELECT count(*) AS prev_enrollees
FROM outreach.enrollees e1
WHERE e1.program_instance_id = 5 AND EXISTS
(SELECT * FROM outreach.enrollees e2
WHERE e1.first_name = e2.first_name
AND e1.last_name = e2.last_name
AND e1.address1 = e2.address1
AND e1.state = e2.state
AND e1.zip = e2.zip
AND e2.program_instance_id < 5);
This query, to my understanding, would give me the number of rows in the 'enrollees' table where an enrollee from the current year (program_instance_id = 5) had previous enrolled in another year. The result it produces is pretty accurate, to my understanding.
So... I ran this EXACT query (changing the table name) on my 'nominations' table. The nominations table has almost the exact structure of the 'enrollees' table (some columns are different, but the person's information fields are identical). This query ran for over a half hour before I cancelled it. It's not popping out an almost-instant result like it was on the enrollee table and I don't know why it would take longer.
I could imagine if there were a lot more rows in the table but the enrollee table has about 50k MORE rows than the nominations table.
I've also tried:
SELECT count(*) AS prev_enrollees
FROM outreach_grow_education.nominations e1
JOIN outreach_grow_education.nominations e2 ON e1.first_name = e2.first_name
AND e1.last_name = e2.last_name
AND e1.address1 = e2.address1
AND e1.state = e2.state
AND e1.zip = e2.zip
AND 4 = e2.program_instance_id
WHERE e1.id IS NOT NULL AND e1.program_instance_id = 2;
Alas, to the same result. Instant result on enrollees, never-ending on nominations.
Is there any other alternative for what I'm trying to achieve that wouldn't cause the never-ending cycle?

Answer 1

I suggest checking the indexes of the two tables, specifically for the columns you use in the JOIN clauses: first_name, last_name, address1, state, zip, and program_instance_id. Chances are one or more of these columns is indexed in the "enrollees" table and not in "nominations."