Primary key column value as two column values combined with different format

Question

So I want to make a primary key that combines the year of a date value followed by - and then a foreign key value that is an order number.

Basically: 2010-244

This is what I tried, (I know I should parse a part of the date string):

ORDERNR int(11) NOT NULL,
RECEIPTDATE DATE,
RECEIPTNR as RECEIPTDATE + "-" + ORDERNR INT NOT NULL UNIQUE,
FOREIGN KEY (ORDERNR) REFERENCES ORDER(ORDERNR),
PRIMARY KEY (RECEIPTNR)) ENGINE=INNODB;

Answer 1

MySQL does not offer function-based indexes (with the minor exception of indexing the leading characters in a string). In other words, you want to create an index on an expression and MySQL does not support that.

If the "year" component of a date is unique, then the data can only have one date per year. So, you can have a unique index on the entire date.

This would be accepted as a primary key:

unique (RECEIPTDATE, ORDERNR);

But this doesn't give you the uniqueness within a year.

Another alternative -- which I don't recommend, but I suggest because it might work -- is to store the date in a string in YYYY-MM-DD format. Then you can do:

unique (RECEIPTDATESTRING(4), ORDERNR)

The alternative is to have a trigger that tests for uniqueness.

Further advice. Don't make a composite primary key, particularly when one of the keys is only part of a column. Instead, use a unique constraint on the fields and have an auto-incremented primary key.