Question

I know the question seems duplicate, but I don’t know how to ask it differently.

I have two very simple tables in MySQL database, The first is table Users

id,   user_id
 1       1
 2       3
 4       4

The second is table Friends

id,    user_id,    friend_id
 1        1             3
 2        1             4
 3        1             8

I dumped the data from CSV file that I would like to clean. I need to check if friend_id exists in table 1 as well. The first table has around 30000 rows, but the second table has around 30 million rows.

And I use this query to check

SELECT u.user_id, uf.friend_id as exists_friend_ids
FROM Users u, Friends uf
WHERE u.user_id = '1'
    and uf.friend_id IN (select user_id from eventify.Users)

However, my desired output would be this but as I cannot run the above query to actually give my test results I cannot continue.

user_id,   exists_friend_ids
    1            3
    1            4

You can see that 8 is not there, because it doesn’t exist in Users table. But as the second table has over 30 million records it’s just running forever on my computer. Am I doing it right or this is the only way to do it. Or should I learn Hadoop instead?

I have updated my query to use equal join.

Answer 1

Have you tried a LEFT JOIN query with a GROUP BY friend_id ? If a user doesn’t exist, it won’t add a line to the result.