How do I select a record from one table in a mySQL database, based on the existence of data in a second?

Question

Please forgive my ignorance here. SQL is decidedly one of the biggest "gaps" in my education that I'm working on correcting, come October. Here's the scenario:

I have two tables in a DB that I need to access certain data from. One is users , and the other is conversation_log . The basic structure is outlined below:

users:

• id (INT)
• name (TXT)

conversation_log

• userid (INT) // same value as id in users - actually the only field in this table I want to check
• input (TXT)
• response (TXT)

(note that I'm only listing the structure for the fields that are {or could be} relevant to the current challenge)

What I want to do is return a list of names from the users table that have at least one record in the conversation_log table. Currently, I'm doing this with two separate SQL statements, with the one that checks for records in conversation_log being called hundreds, if not thousands of times, once for each userid, just to see if records exist for that id.

Currently, the two SQL statements are as follows:

select id from users where 1; (gets the list of userid values for the next query)

select id from conversation_log where userid = $userId limit 1; (checks for existing records)

Right now I have 4,000+ users listed in the users table. I'm sure that you can imagine just how long this method takes. I know there's an easier, more efficient way to do this, but being self-taught, this is something that I have yet to learn. Any help would be greatly appreciated.

Answer 1

You have to do what is called a 'Join'. This, um, joins the rows of two tables together based on values they have in common.

See if this makes sense to you:

SELECT DISTINCT users.name
FROM users JOIN conversation_log ON users.id = converation_log.userid

Now JOIN by itself is an "inner join", which means that it will only return rows that both tables have in common. In other words, if a specific conversation_log.userid doesn't exist, it won't return any part of the row, user or conversation log, for that userid.

Also, +1 for having a clearly worded question : )

EDIT: I added a "DISTINCT", which means to filter out all of the duplicates. If a user appeared in more than one conversation_log row, and you didn't have DISTINCT, you would get the user's name more than once. This is because JOIN does a cartesian product , or does every possible combination of rows from each table that match your JOIN ON criteria.

Answer 2

Something like this:

SELECT *
FROM users
WHERE EXISTS (
    SELECT *
    FROM conversation_log
    WHERE users.id = conversation_log.userid
)

In plain English: select every row from users , such that there is at least one row from conversation_log with the matching userid .

Answer 3

What you need to read is JOIN syntax.

SELECT count(*), users.name 
FROM users  left join conversion_log  on users.id = conversation_log.userid
Group by users.name

You could add at the end if you wanted HAVING count(*) > 0