SQL auto_increment non-int-only values - mysql

Question

I was looking into auto_increment and I would like to automatically generate unique id primary key. The problem is I only know how to do that with numerical ids. As far as I know, primary keys have to be unique. So I cant use numerical auto_increment for multiple tables because the id wont be unique. Here's how I do it now:
CREATE TABLE students (
id int(3) NOT NULL AUTO_INCREMENT PRIMARY KEY,
name char(20),
lastname char(20)
);
To the best of my knowledge this will generate ids like: 1, 2, 3, 4 unless I name a different starting number. Would it be possible to autoi_increment something like "stud_1" or "stud_01" and then keep going increasing the number? I haven't come across such an example so far.

Answer 1

I wrote a lot in the comments above, but let me apply it to a pratical application in an answer. We'll begin with your table and then add two more:
Tables:
CREATE TABLE students (
id int(3) NOT NULL AUTO_INCREMENT PRIMARY KEY,
name char(20),
lastname char(20)
) ENGINE = INNODB;
CREATE TABLE subjects (
id int(3) NOT NULL AUTO_INCREMENT PRIMARY KEY,
name char(20)
) ENGINE = INNODB;
CREATE TABLE student_subject (
student_id int(3) NOT NULL,
subject_id int(3) NOT NULL,
PRIMARY KEY(student_id, subject_id)
FOREIGN KEY(student_id) references students(id),
FOREIGN KEY(subject_id) references subjects(id)
) ENGINE = INNODB;
With a setup of tables like this, it is possible to have multiple numerical auto_increment primary key columns within the same database. In this case, there are two students.id and subjects.id. The third table student_subject shows how to link these together where both student_id and subject_id are primary keys for that table but each is a foreign key of their entity table as well.
With a setup like this querying becomes simple:
Sample Queries:
/* Get All students */
SELECT
students.id,
students.name,
students.lastname
FROM students;
/* Get All Subjects */
SELECT
subjects.id,
subjects.name
FROM subjects;
/* Get all subjects for a particular student */
SELECT
subjects.name
FROM subjects
INNER JOIN student_subject ON subjects.id = student_subject.subject_id
INNER JOIN students ON student_subject.student_id = students.id
WHERE students.id = 1;
EDIT:
Below is some sample data that demonstrates the three table many-to-many concept that the above structure illustrates:
/* Students Table */
+----+----------------+----------------+
| id | name | lastname |
+----+----------------+----------------+
| 1 | John | Doe |
|----|----------------|----------------|
| 2 | Joe | Bob |
+----+----------------+----------------+
/* Subjects Table */
+----+----------------+
| id | name |
+----+----------------+
| 1 | Calculus |
|----|----------------|
| 2 | Physics |
|----|----------------|
| 3 | Geography |
+----+----------------+
/* Student Subject Table */
+------------+------------+
| student_id | subject_id |
+------------+------------+
| 1 | 1 |
|------------|------------|
| 1 | 2 |
|------------|------------|
| 2 | 2 |
|------------|------------|
| 2 | 3 |
+------------+------------+
With this table structure, each student can have many subjects and each subject can belong to many students. For example, John Doe (student 1) is enrolled in both Calculus (subject 1) and Physics (subject 2). Joe Bob (student 2) is enrolled in Geography (subject 3) and also in Physics (subject 2) with John Doe.
In this example, each student is enrolled in two subjects and both students are enrolled in one subject. With the example you linked to from w3schools.com, this many-to-many relationship would not be possible.

Answer 2

Primary key values only need to be unique within a table, so there's no reason you can't use simple auto-increment numeric ids for every table.
As far as the question - I don't believe there's a direct way to "auto-increment" a string value in the way you describe. You could set something up with triggers and a sequence (or regular auto-increment column) if you really wanted to.