Recover indexes primary key - mysql

Question

which middle safer for mes to recover and Sort indexes primary key, avoiding unnecessary update throughout the database? Can you show me any examples?
I just wanna that primary keys were constant and dynamic.I just wanna that primary keys were constraint and dinamic
ps: MySQL Database
User's Table
|| user_id || user_first_name || user_last_name
#1 Alexandre Doria
#2 Ilya Bursov
#3 Anybody Anybody
So, if i DELETE the #2 row, and INSERT a new row, the primary key of #3 row is duplicated.
My PHP code here:
$user_id_cont = mysql_query("select 'user_id' from user");
$user_id = mysql_num_rows($user_id_cont)+1;

Answer 1

It absolutely wrong to calculate primary keys as it done in your code
First of all - change user_id column to be primary key (it will be unique too then), also set it to be auto_increment, so your table create statement must be:
create table sample_users (
`user_id` int unsigned not null auto_increment,
`first` varchar(255),
`last` varchar(255),
primary key (`user_id`)
);
next, you need to insert data into your table, so you can just insert it:
INSERT into `sample_users` (`first`, `last`) values('fname', 'lname');
INSERT into `sample_users` (`first`, `last`) values('fname2', 'lname2');
...
mysql will generate appropriate user_id for each row, and even if you delete some of them - mysql still will provide you with unique values
additional case is - if you need to know what id is generated by mysql, you can use PHP mysql_insert_id or MySQL LAST_INSERT_ID functions for that
just for notice: wrong method, but it will work for "school project", change your code to:
$user_id_cont = mysql_query("select max('user_id') from user");
$user_id = mysql_fetch_array($user_id_cont, MYSQL_NUM);
$user_id = $user_id[0]+1;