INSERT new row or DELETE old row IF it already exists

Question

I need to optimize/implement a MYSQL query wich do the following:

I need to store if an user marked an item as seen, and create a SEEN/UNSEEN button, that delete the correspondent row on the database if already exists (is marked as seen) OR insert a new row if it not (is not marked).

It means, I need a mysql query that do that:

    DELETE FROM table WHERE userid = ? AND itemid = ?

or that

    INSERT INTO table (userid, itemid) VALUES (?,?)

depending on if the row already exists or not.

I now it's very easy doing it with php and checking the number of rows of a SELECT before INSERT or DELETE but I would like to doing that all the optimized I can.

Answer 1

You can use stored procedure to SELECT and DELETE if exists or INSERT. But much easier would just be left the data there and update seen/unseen with ON DUPLICATE KEY UPDATE.

Answer 2

You could use a third column to store an integer, ad use that value as a "toggle" to switch between seen/unseen through an UPSERT query.

Something like:

INSERT INTO table (userid, itemid, seen) VALUES (?,?,0) ON DUPLICATE KEY UPDATE seen = ((seen+1) MOD 2);

The primary key should be formed by userid and itemid. The MOD operator will make it easy and efficient to implement the seen/unseen behaviour you want.

Answer 3

IMHO, that's a wrong idea. The button in UI shouldn't be called "Make seen/unseen". The button in UI must have concrete name, so user will know what he is doing. So, you will need to make SELECT before displaying a button anyway. And button click event algorithm should not be like this:

if not currently_is_seen then mark_as_seen else mark_as_not_seen;

Button algorithm should be like this:

if current_button_name_is_mark_seen then {
    if not currently_is_seen then mark_as_seen;
} else {
    if currently_is_seen then mark_as_not_seen;
}

(It's ok if under some specific conditions button is named "mark seen" althrough it's already marked seen -- button will just do nothing. It's ok if under some specific conditions button is named "mark unseen" althrough it's already marked unseen -- button will just do nothing. But it's not ok when button titled "mark seen" or "mark unseen" does an opposite action instead.)

To implement

if not currently_is_seen then mark_as_seen;

and

if currently_is_seen then mark_as_not_seen;

you can use

INSERT IGNORE INTO table (userid, itemid) VALUES (?,?)

and

DELETE FROM table WHERE userid = ? AND itemid = ?

(delete doesn't cause error if record doesn't exists).

Hovewer, if you want to implement the

if not currently_is_seen then mark_as_seen else mark_as_not_seen;

logic very much, you can use stored routine (also here).

Answer 4

INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6) ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b); check the following link

http://www.tutorialspoint.com/mysql/mysql-handling-duplicates.htm