Preventing duplicate rows in mysql db, IGNORE not working - php

Question

I am trying to prevent the insertion of duplicate rows into mysql database. The table name is favorites and I have two columns in my table: company_id and user_id, I want to prevent users to try to add the same company to the db as a 'favorite' twice.
This is what I tried:
$query = "INSERT IGNORE INTO favorites (item_id, user_id) VALUES ( $item_id, $user_id )";
mysql_query($query,$conn);
But does not work.
I also tried to 'alter table' to add a primary key, however, I need both user_id and item_id to be keys, because the same favorited item can be favorited by more than one 'user_id' and the same 'user_id' can insert many different favorited items, so that data can be 'duplicated' but I am trying to prevent the exact same 'user_id' and 'item_id' to be inserted twice.
I appreciate any help with this.

Answer 1

The easiest way I know of is to add a UNIQUE constraint on the user_id-item_id pair, which would be accomplished with the following query:
ALTER TABLE favorites
ADD UNIQUE(item_id,user_id)
Your insert query would then return an error whenever you tried to insert a user_id-item_id pair that already existed in your table, so your INSERT query should be modified thusly:
INSERT INTO favorites(item_id,user_id)
VALUES ($item_id,$user_id)
ON DUPLICATE KEY UPDATE item_id=item_id
I do not recommend using "INSERT IGNORE" because it ignores ALL errors. My query will simply set item_id=item_id (no change) whenever it detects a duplicate key, so data will not be duplicated.
I also strongly encourage you to look into using MySQLi instead of the mysql_* functions. The PHP that you posted is very susceptible to mysql injections should you forget to check those two user input variables. Even the PHP manual actively discourages those functions for the same reason.

Answer 2

You can use a composite primary key of the columns like so:
ALTER TABLE table ADD PRIMARY KEY ( 'item_id' , 'user_id' )
This means that same user_ids and item_ids are allowed and only a combination of them needs to be unique.

Answer 3

Try adding a composite primary key (from item_id and user_id) to the table.
You will get an error if you already have data in your table that would violate this constraint, in this case you will need to create a new table and migrate your data into the new table.

Answer 4

INSERT IGNORE only works on keys. Typically the only key you have is the primary key on a table.
So, first search for matching rows and if they exist, don't insert a new record.
$search_q = "SELECT `id` FROM `favorites` WHERE `item_id` = ";
$search_q .= mysql_real_escape_string($item_id);
$search_q .= " AND `user_id` = ";
$search_q .= mysql_real_escape_string($user_id);
$r = mysql_query($search_q);
if (!mysql_num_rows($r)) {
# This combination doesn't exist
$insert_q = "INSERT INTO `favorites` (`item_id`,`user_id`) VALUES (";
$insert_q .= mysql_real_escape_string($item_id).",";
$insert_q .= mysql_real_escape_string($user_id).")"
mysql_query($insert_q);
}