MYSQL Table creation with default value(expression) to a column - mysql

Question

I have a Table Employee(id,name,dept_name).I want the id will alphanumeric [dddddaaaaa] with first 5 digit will be auto increment id and rest 4 char will be the first 4 char of employee name.
For example , for the first employee name=John Todd ,the auto incremented part of the Id will be 00001. And so the Id will be 00001JOHN.
Is it possible to set a default expression to the column Id=(concat(autoincrement,substring(name,4)).
I was also thinking if I Can create a trigger on after insert Employee and the trigger will update the Employee.Id. But MySql does not allow to update the same table from trigger for which trigger got fired.
Please Help.

Answer 1

What about a schema like
CREATE TABLE employee
(
employeeid INT PRIMARY KEY AUTO_INCREMENT,
firstname varchar(255)
);
CREATE INDEX part_of_firstname ON employee (firstname(4));
That'll let you perform lookups fairly quickly using your natural primary key, while giving you an artificial primary key and not forcing to denormalize.
EXPLAIN SELECT * FROM EMPLOYEE WHERE EMPLOYEEID = 1 AND FIRSTNAME LIKE 'john%';
+----+-------------+----------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | employee | const | PRIMARY,part_of_firstname | PRIMARY | 4 | const | 1 | |
+----+-------------+----------+-------+---------------------------+---------+---------+-------+------+-------+
Of course since the 0001 part of the primary key is unique enough to identify the user you need not query the name at all.
If you insist on precalculating this should work
CREATE TABLE employee
(
employeeid INT PRIMARY KEY AUTO_INCREMENT,
specialid VARCHAR(255),
firstname VARCHAR(255)
);
CREATE INDEX employee_specialid ON employee (firstname(4));
DELIMITER ;;
CREATE TRIGGER employeeid_trigger BEFORE insert ON employee
FOR EACH ROW
BEGIN
SET new.specialid = CONCAT(LPAD((SELECT AUTO_INCREMENT FROM information_schema.TABLES WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = 'employee'), 4, '0'), SUBSTRING(new.firstname, 1, 4));
END
;;
DELIMITER ;
Testing it:
mysql> insert into employee (firstname) values ('johnathan');
Query OK, 1 row affected (0.04 sec)
mysql> insert into employee (firstname) values ('johnathan');
Query OK, 1 row affected (0.02 sec)
mysql> insert into employee (firstname) values ('johnathan');
Query OK, 1 row affected (0.02 sec)
mysql> select * from employee;
+------------+-----------+-----------+
| employeeid | specialid | firstname |
+------------+-----------+-----------+
| 1 | 0001john | johnathan |
| 2 | 0002john | johnathan |
| 3 | 0003john | johnathan |
+------------+-----------+-----------+
3 rows in set (0.00 sec)
This is kind of a hack, and information_schema won't be available on some DBs where permissions aren't under your control.

Answer 2

You could try concatenating it in your select statement instead of storing an auto increment column and an id column,
SELECT CONCAT(id, substring(name,4) FROM tbl_employee
That way you wouldn't need triggers