Question

This question here:

MySQL: INSERT IGNORE or ON DUPLICATE KEY UPDATE with checking multiple and not unique columns

Is similar, but not quite.

I am trying to build a watch list, using a table as follows:

    recordId    |    itemId   |   userId    |    dateAdded

The recordId is the only unique column and ss an autoincrement ID field.

itemId, userId and dateAdded can be duplicated individually, but not altogether.

So, if a user has already added an item to his/her watch list, I want it to just update the dateAdded.

Rows therefore may contain the same userId many times (as the user may wish to ‘watch’ multiple items) and it may include the itemId many times (as the same item may be ‘watched’ by many users) but a combination of both the userId and itemId matching a record which already exists should cause an update.

Is there a single statement way to do this, or do I need to ask the database if userId = & postingUser & and itemId = & requestedId & before attempting an insert?

Answer 1

Here’s how you can do this:

insert into <tablename> (itemId, userId)
values (?, ?)
on duplicate key update dateAdded = now();

where the ?’s stand for the new values you’re inserting

This is assuming you have defined a unique index for (itemId, userId):

CREATE TABLE <table> (
  `recordId` int(11) NOT NULL AUTO_INCREMENT,
  `itemId` int(11) DEFAULT NULL,
  `userId` int(11) DEFAULT NULL,
  `dateAdded` timestamp,
  PRIMARY KEY (`recordId`),
  UNIQUE KEY itemuser (`itemId`,`userId`)
) ENGINE=InnoDB;

How this works:

• when you try to insert a row, MySQL will check for key conflicts
• since you’ve created a unique key for the combination of (itemId, userId), this will be checked
• if there is no conflict, MySQL will go ahead and insert the row
• if there is a conflict, MySQL will fall back to the on duplicate key clause, which updates the dateAdded column’s value