Is it possible to CREATE TABLE with a column that is a combination of other columns in the same table? - sql

Question

I know that the question is very long and I understand if someone doesn't have the time to read it all, but I really wish there is a way to do this.
I am writing a program that will read the database schema from the database catalog tables and automatically build a basic application with the information extracted from the system catalogs.
Many tables in the database can be just a list of items of the form
CREATE TABLE tablename (id INTEGER PRIMARY KEY, description VARCHAR NOT NULL);
so when a table has a column that references the id of tablename I just resolve the descriptions by querying it from the tablename table, and I display a list in a combo box with the available options.
There are some tables however that cannot directly have a description column, because their description would be a combination of other columns, lets take as an example the most important of those tables in my first application
CREATE TABLE bankaccount (
bankid INTEGER NOT NULL REFERENCES bank,
officeid INTEGER NOT NULL REFERENCES bankoffice,
crc INTEGER NOT NULL,
number BIGINT NOT NULL
);
this as many would know, would be the full account number for a bank account, in my country it's composed as follows
[XXXX][XXXX][XX][XXXXXXXXXX]
^ ^ ^ ^
bank id | crc account number
|
|_ bank office id
so that's the reason of the way my bankaccount table is structured as is.
Now, I would like to have the complete bank account number in a description column so I can display it in the application without giving a special treatment to this situation, since there are some other tables with similar situation, something like
CREATE TABLE bankaccount (
bankid INTEGER NOT NULL REFERENCES bank,
officeid INTEGER NOT NULL REFERENCES bankoffice,
crc INTEGER NOT NULL,
number BIGINT NOT NULL,
description VARCHAR DEFAULT bankid || '-' || officeid || '-' || crc || '-' || number
);
Which of course doesn't work since the following error is raised1
ERROR: cannot use column references in default expression
If there is any different approach that someone can suggest, please feel free to suggest it as an answer.
1 This is the error message given by PostgreSQL.

Answer 1

What you want is to create a view on your table. I'm more familiar with MySQL and SQLite, so excuse the differences. But basically, if you have table 'AccountInfo' you can have a view 'AccountInfoView' which is sort of like a 'stored query' but can be used like a table. You would create it with something like
CREATE VIEW AccountInfoView AS
SELECT *, CONCATENATE(bankid,officeid,crc,number) AS FullAccountNumber
FROM AccountInfo
Another approach is to have an actual FullAccountNumber column in your original table, and create a trigger that sets it any time an insert or update is performed on your table. This is usually less efficient though, as it duplicates storage and takes the performance hit when data are written instead of retrieved. Sometimes that approach can make sense, though.

Answer 2

What actually works, and I believe it's a very elegant solution is to use a function like this one
CREATE FUNCTION description(bankaccount) RETURNS VARCHAR AS $$
SELECT
CONCAT(bankid, '-', officeid, '-', crc, '-', number)
FROM
bankaccount this
WHERE
$1.bankid = this.bankid AND
$1.officeid = this.officeid AND
$1.crc = this.crc AND
$1.number = this.number
$$ LANGUAGE SQL STABLE;
which would then be used like this
SELECT bankaccount.description FROM bankaccount;
and hence, my goal is achieved.
Note: this solution works with PostgreSQL only AFAIK.