Creating Primary key from 2 autonumber and constant letter when creating table - mysql

Question

I am new to MYSQL and would like to create a table where a constant Letter depicting the department is added to an auto increment number. This way I would be able to identify the category of the worker upon viewing the ID.
Ex. Dept A and employee 135. The ID I am imaging should read A135 or something similar. I have created the table, the auto increment works fine, the constant letter has been declared and is featuring. However I would like to concatenate them in order to use the A135 as a primary key.
Any Help Please?

Answer 1

This quite tricky, and you would be probably better off doing manual concatenation in a select query.
But since you asked for it...
In normal usage you would have used a computed column for this, but they do not support using autoincremented columns in their declaration. So you would need to use triggers:
on insert, query information_schema.tables to retrieve the autoincremented id that is about to be assigned and use it to generate the custom id
on update, reset the custom id
Consider the following table structure:
create table workers (
id int auto_increment primary key,
name varchar(50) not null,
dept varchar(1) not null,
custom_id varchar(12)
);
Here is the trigger for insert:
delimiter //
create trigger trg_workers_insert before insert ON workers
for each row
begin
if new.custom_id is null then
select auto_increment into #nextid
from information_schema.tables
where table_name = 'workers' and table_schema = database();
set new.custom_id = CONCAT(new.dept, lpad(#nextid, 11, 0));
end if;
end
//
delimiter ;
And the trigger for update:
delimiter //
create trigger trg_workers_update before update ON workers
for each row
begin
if new.dept is not null then
set new.custom_id = CONCAT(new.dept, lpad(old.id, 11, 0));
end if;
end
//
delimiter ;
Let's run a couple of inserts for testing:
insert into workers (dept, name) values ('A', 'John');
insert into workers (dept, name) values ('B', 'Jim');
select * from workers;
| id | name | dept | custom_id |
| --- | ---- | ---- | ------------ |
| 1 | John | A | A00000000001 |
| 2 | Jim | B | B00000000002 |
And let's test the update trigger
update workers set dept = 'C' where name = 'Jim';
select * from workers;
| id | name | dept | custom_id |
| --- | ---- | ---- | ------------ |
| 1 | John | A | A00000000001 |
| 2 | Jim | C | C00000000002 |
Demo on DB Fiddle

Answer 2

Sorry, my answer does not fit in a comment.
I agree with #GMB.
This is a tricky situation and in some cases (selects mainly) will lead in a performance risk due you'll have to split PK in where statements, which is not recommended.
Having a column for department and another for auto_increment is more logical. And the only gap you have is to know the number of employees per department you'll have to make a count grouping by dept. Instead of a max() splitting your concatenated PK, which is is at high performance cost.
Let atomic and logic data remain in separate columns. I would suggest to create a third column with the concatenated value.
If, for some company reason, you need B1 and A1 values for employees of different departments, I'd suggest to have 3 columns
Col1 - letter(not null)
Col2 - ID(Not auto-increment, but calculated as #GMB's solution) (Not NULL)
Col3 - Concatenation of Col1 and Col2 (not null)
PK( Col1, col2)