Inner Join versus Exists() while avoiding duplicate rows - sql

Question

This is a complicated question so bear with me as I set up the scenario:
Say we have a simplified table setup like so:
table 1(employee): {
employee_id, -primary key
first_name,
last_name,
days_of_employment
}
with data:
employee_id first_name last_name days_of_employment
111 Jack Stevens 543
222 Clarice Bobber 323
333 Roy Cook 736
444 Fred Roberts 1000
...
table 2(teams): {
team_code, --primary key
description
}
with data:
team_code description
ERA Enrollment Records Assoc.
RR Rolling Runners
FR French Revolution
...
table 3(employees_teams):{
employee_id, --primary key
team_code --primary key
}
with data:
employee_id team_code
111 RR
111 FR
222 FR
222 ERA
333 FR
...
I'm hoping these tables should be clear as to what they are and their purpose. Here is my scenario from requirements: "I want the average days of employment of employees on the Rolling Runners and Enrollment Records Assoc. team." There are two ways I know how to write this query and they both seem to work well enough, but what I really want to know is which one is faster for the oracle database to process. Keep in mind that these queries are written the way they are to keep from producing duplicate rows which would screw up the average calculation:
Query 1:
SELECT AVG(e.days_of_employment) avg_days_of_employment
FROM employee e,
(
SELECT DISTINCT employee_id
FROM employees_teams
WHERE team_code IN ('ERA','RR')) available_employees
WHERE e.employee_id = available_employees.employee_id
Query 2:
SELECT AVG(e.days_of_employment) avg_days_of_employment
FROM employee e
WHERE EXISTS(
SELECT 1
FROM employees_teams et
WHERE et.team_code IN ('ERA','RR')
AND et.employee_id = e.employee_id)
It is possible that with this sample data I provided that this situation may not make sense to begin with, but I still would like to know which query is 'better' to use.

Answer 1

I would say go with the EXISTS approach since you are not really needing anything from the available_employees other than checking for the existence.
Having said that it depends on your data as well and how your database query optimizer optmizes it. I would suggest you to see the query plan for each approach and see which one is less expensive.
Check these links as well http://dotnetvj.blogspot.com/2009/07/why-we-should-use-exists-instead-of.html Can an INNER JOIN offer better performance than EXISTS