MySQL if row exists update else insert

Question

I'm trying to make a site where people can sign up for events + an activity assigned to that event. They can choose which one they want to attend, or even both.

For that I made a table named "eventCounter" Image of table
userID, eventID and activityID are all FK poiting to other tables.

They should be able to update their current "status", so they can join the activity after they signed up for the event.

So my question is: How can I make a If else saying if row exists update else insert

IF EXISTS(select userID, eventID, activityID from eventCounter where userID=1 and eventID=1)
THEN 
UPDATE eventcounter SET activityID=1 WHERE userID=1; 
ELSE 
INSERT INTO eventcounter (userID, activityID) VALUES(1,1)

I don't think the ON DUPLICATE key will work as I have 2 columns that needs to be checked?

Answer 1

That's what is called an Upsert. The MySQL syntax is pretty weird, but you can do it, something like:

INSERT INTO eventcounter (userID, eventID, activityID) VALUES(1,1,1)
ON DUPLICATE KEY UPDATE
  activityID = VALUES(activityID)

It only works for duplicate PKs though (not for any field you like), so in this case it would only work if userID and eventID compose the PK and you only want to change the activityID

Otherwise, you could go the IF-ELSE route, something like:

DECLARE mycount INT;
SET mycount = (SELECT COUNT(*) FROM eventcounter WHERE userID=1 AND eventID=1);
IF mycount > 0 THEN
  UPDATE eventcounter SET activityID=1 WHERE userID=1 AND eventID=1;
ELSE
  INSERT INTO eventcounter (userID, eventID, activityID) VALUES(1,1,1);
END IF;

Disclaimer: totally untested code

Answer 2

As 2021, things changed! So i'm updating the old @Jcl's answer:

One thing you have to bear in mind is that this statement only works if a duplication occurs either to PRIMARY KEY or UNIQUE INDEX in case of inserting (so not only PK but index also works)!

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SQL command:

INSERT INTO fooTable (RowNum, name, lastname, userID, score) VALUES(3,'Michael','Widenius','1012141618',19.75)
ON DUPLICATE KEY UPDATE name = 'Allan', lastname = 'Larsson', userNum = '2022242628', score = 10.50;

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Note:

• The use of VALUES() to refer to the new row and columns is deprecated (Reference)
• Command tested on latest MYSQL 8.0 and PHP Mysqli too.
• Command above is only to show you, how to update multiple rows; in a real world situation, mostly you don't need to update all name, lastname and userID in case that record is already exist and score would be enough!

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I found it pretty straight and easy to perform, you can check duplication on almost every important field in your case (using UNIQUE INDEXs)! just remember the very first line...