MYSQL - Select Unique Common Columns between two tables - Most Efficient Query - mysql

Question

I have two tables:
db_contacts
Phone | Name | Last_Name
--------------------
111 | Foo | Foo
222 | Bar | Bar
333 | John | Smith
444 | Tomy | Smith
users_contacts
User_ID | Phone
--------------------
1 | 123
1 | 111
2 | 222
2 | 333
3 | 111
3 | 333
4 | 444
Notice from above that:
User with ID 2 is the only one that have the phone number 222
User with ID 4 is the only one that have the phone number 444
I need to obtain these results with a MySQL query.
In other words: How can I select all the users that have a unique phone number in condition that this number exists in the db_contacts.
I need my end result to be something like that:
User_ID | Phone | Name | Last_Name
------------------------------------
2 | 222 | Bar | Bar
4 | 444 | Tomy | Smith
PS: There is no Foreign key between the Phone columns, as a User can have a phone that is not in the db_contacts.
In real life, db_contacts contains about 1 million records and users_contacts about 5 million records.
What I tried and failed and taking a lot of time to execute:
SELECT *
FROM users_contacts
WHERE users_contacts.phone IN (
SELECT users_contacts.phone
FROM `users_contacts`
JOIN db_contacts ON db_contacts.phone = users_contacts.phone
GROUP BY users_contacts.phone
HAVING COUNT(users_contacts.phone) = 1
)
Update:
Thank you for your replies, I have provided my solution that fits my case perfectly.

Answer 1

I think you want:
select uc.*
from user_contacts uc
where not exists (select 1
from user_contacts uc2
where uc2.phone = uc.phone and uc2.user_id <> uc.user_id
);
For performance, you want an index on user_contacts(phone, user_id).
Another method is:
select max(user_id) as user_id, phone
from user_contacts
group by phone
having count(*) = 1;
The not exists version is probably going to be faster.

Answer 2

I would use a simple JOIN with a NOT EXISTS condition. This is usually the most efficient way to check that something has no duplicates ; compared to your solution, this has the advantage of avoiding aggregation.
SELECT uc.User_ID, dc.*
FROM users_contacts uc
INNER JOIN db_contacts dc ON uc.Phone = dc.Phone
WHERE NOT EXISTS (
SELECT 1
FROM users_contacts uc1
WHERE uc1.Phone = dc.Phone AND uc1.User_ID != uc2.User_ID
)
Hint: consider setting the following indexes:
users_contacts(Phone, User_ID)
db_contacts(Phone)

Answer 3

I first would like to thank everyone that posted solutions, they all worked.
But I was a bit crucial on response times, and solutions provided by the fellows took a lot of time to execute, couple of seconds.
In case anyone was having a similar problem, I ended up by creating a new table calling it users_unique_contacts, and created a trigger AFTER INSERT on users_contacts that checks if the newly created contact existed in the users_unique_contacts, if it didn't exist, add it, else remove it as it means the number is not unique anymore.
My Trigger went like this:
BEGIN
IF EXISTS (SELECT 1 = 1 FROM users_unique_contacts WHERE phone = new.phone LIMIT 1) THEN
BEGIN
DELETE FROM users_unique_contacts WHERE phone = new.phone LIMIT 1;
END;
ELSE
BEGIN
INSERT INTO users_unique_contacts (user_id,phone) VALUES (new.user_id, new.phone);
END;
END IF;
END
Now everytime I want the unique numbers of a user, I query the users_unique_contacts and execution time is milliseconds.