Informix - Create table with function weekday - sql

Question

I'm trying to create a table on Informix (11.70). I want to add the function WEEKDAY to a datetime value on the CREATE clause, so that I can automatically get an integer returned when inserting data.
For example.
CREATE TABLE orders (
order_num serial
order_date datetime year to second
order_weekday datetime year to second
)
I've tried the following and I get syntax error.
CREATE TABLE orders (
order_num serial
order_date datetime year to second
order_weekday WEEKDAY(datetime) year to second
)
Also this
CREATE TABLE orders (
order_num serial
order_date datetime year to second
WEEKDAY(order_weekday) datetime year to second
)
Is there some way to achieve this?

Answer 1

What are you trying to achieve? It is a fundamental principle of RDBMS normalisation that you should not store a value that can be derived from another existing field. You can calculate WEEKDAY(order_date) any time you need it at virtually no cost.
However, if you really want to do this, what you need to do is write an INSERT TRIGGER, as #JonathanLeffer suggested whilst I was writing this.
You might also want to clarify what you mean by "get an integer returned". The database returns something on a successful INSERT, but you cannot override that arbitrarily. If you really want to do that, you need to write a PROCEDURE to handle inserting the record to the orders table and returning the value you require.

Answer 2

You best bet is probably using FOR EACH ROW triggers which assign to the weekday column on INSERT and UPDATE given the value in the Order-Date column.
As RET notes in his answer, it is often best not to store derived data, especially not such readily derivable data as the weekday.
However, assuming you need to do so, then your trigger would look like:
BEGIN WORK;
CREATE TABLE orders
(
order_num SERIAL NOT NULL,
order_date DATETIME YEAR TO SECOND NOT NULL,
order_weekday INTEGER CHECK (order_weekday BETWEEN 0 AND 6) NOT NULL
);
INSERT INTO orders VALUES(0, CURRENT YEAR TO SECOND, WEEKDAY(MOD(WEEKDAY(TODAY) + 3, 7)));
SELECT *, WEEKDAY(order_date) AS calc_weekday FROM orders;
order_num order_date order_weekday calc_weekday
SERIAL DATETIME YEAR TO SECOND INTEGER SMALLINT
1 2016-02-04 23:21:36 0 4
CREATE TRIGGER i_orders INSERT ON orders
REFERENCING NEW AS NEW
FOR EACH ROW
(
UPDATE orders
SET order_weekday = WEEKDAY(NEW.order_date)
WHERE order_num = NEW.order_num
);
CREATE TRIGGER u_orders UPDATE OF order_date ON orders
REFERENCING NEW AS NEW
FOR EACH ROW
(
UPDATE orders
SET order_weekday = WEEKDAY(NEW.order_date)
WHERE order_num = NEW.order_num
);
INSERT INTO orders(order_num, order_date) VALUES(0, CURRENT YEAR TO SECOND);
SELECT *, WEEKDAY(order_date) AS calc_weekday FROM orders;
order_num order_date order_weekday calc_weekday
SERIAL DATETIME YEAR TO SECOND INTEGER SMALLINT
1 2016-02-04 23:21:36 0 4
2 2016-02-04 23:21:36 4 4
UPDATE orders
SET order_date = order_date - 10 UNITS DAY
WHERE order_weekday != WEEKDAY(order_date);
SELECT *, WEEKDAY(order_date) AS calc_weekday FROM orders;
order_num order_date order_weekday calc_weekday
SERIAL DATETIME YEAR TO SECOND INTEGER SMALLINT
1 2016-01-25 23:21:36 1 1
2 2016-02-04 23:21:36 4 4
ROLLBACK WORK;