How to select a multiple column primary key of an inserted row? - php

Question

Problem:
I have the following table in MySQL.
For this example lets say that there is (and always will be) only one person in the world called "Tom" "Bell". So (name, surname) is the PRIMARY KEY in my table. Every person has his salary, an unsigned integer.
CREATE TABLE IF NOT EXISTS `user` (
`name` varchar(64) NOT NULL DEFAULT 'Default_name',
`surname` varchar(64) NOT NULL DEFAULT 'Default_surname',
`salary` int(10) unsigned NOT NULL DEFAULT '0',
UNIQUE KEY `id` (`name`,`surname`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
Whenever I insert a row using a PHP script I want my function to return the primary key of the inserted row (an array key=>value).
From PHP context I do not know what the primary key of table 'user' consists of and I do not always need to set all primary key values (example 2, very stupid, but possible).
I can add another argument to my insert function (for example I could pass the table name, in this case "user").
If this matters, I am using PDO (php data objects) to connect with my MySQL database.
Example 1:
$db->insert('INSERT INTO `user` (`name`,`surname`,`salary`) VALUES ('Tom','Bell','40');');
should return an array:
$arr = ['name' => 'Tom', 'surname' => 'Bell'];
Example 2:
$db->insert('INSERT INTO `user` (`name`,`salary`) VALUES ('Nelly','40');');
should return an array:
$arr = ['name' => 'Nelly', 'surname' => 'Default_surname'];
Disclaimer & other information:
I know this is not a well-designed table, I could use an auto_increment id column to make it much easier and probably more efficient as well. This is just an example to show the problem without having to explain my project structure.
Without loss of generality: Using functions like "getLastInsertId()" or "##identity" will return 0, I guess the reason is because the table does not have an auto_increment column.
What have I tried? Nothing (other than things stated in point 2 (which I was certain it wouldn't work) and searching for a solution).

Answer 1

There aren't "nice" ways around this problem. One of the reasons for having an auto_increment is to avoid having problems like you described.
Now, to avoid my answer to be one of those that take into account only half the picture - I do realize that sometimes you inherit a project or you simply screw up during initial stages and you have to fix things quickly.
To reflect on your example - your PK is a natural PK, not a surrogate one like auto_increment is. Due to that fact it's implied that you always know the PK.
In your example #1 - you inserted Tom Bell - that means you knew the PK was Tom Bell since you instructed MySQL to insert it. Therefore, since you knew what the PK was even before insert, you know how to return it.
In your example #2 you specified only a part of the PK. However, your table definition says thtat default values for both name and surname are Default_surname. That means, if you omit either part of the PK, you know it'll assume the default value. That also means you already know before insertion what the PK is.
Since you have to use a natural PK instead of a surrogate, the responsibility of "knowing" it shifts to you instead of RDBMS. There is no other way of performing this action. The problem becomes even more complex if you allow for a default value to become null. That would let you insert more than 1 Tom with null as surname, and the index constraint wouldn't apply (null is not equal to null, therefore (tom, null) is not equal to (tom, null) and insert can proceed).
Long story short is that you need a surrogate PK or the auto_increment. It does everything you require based on the description. If you can't use it then you have a huge problem at your hands that might not be solvable.