Question

Having a following table:

CREATE TABLE `foo`(
  `year` INT NOT NULL,
  `month` INT NOT NULL,
  `day` INT NOT NULL,
  `hour` INT NOT NULL,
  `minute` INT NOT NULL,
  `value` INT NOT NULL,
  PRIMARY KEY(`year`, `month`, `day`, `hour`, `minute`)
);

I want to write a query which will add 2 columns for every record: date for standard single-column data representation and weekday to indicate day-of-a-week number.

I have tried

SELECT
    `year`, `month`, `day`, `hour`, `minute`, `value`,
    substr('000' || year, -4) || '-' || substr('0' || month, -2) || '-' || substr('0' || day, -2) AS `date`,
    strftime('%w', `date`) AS `weekday`
FROM
    `foo`;

But this says

Error: no such column: date

This is an illustration. In real I have much more complex logic in calculating additional columns and need to reuse these calculated columns to calculate other columns and just copying whole code in every place I need its value look quite scary.

Is there a way I can address a computed column value in computing another one?

Answer 1

Use a subquery:

select f.*, strftime('%w', `date`) AS `weekday`
from (SELECT `year`, `month`, `day`, `hour`, `minute`, `value`,
             substr('000' || year, -4) || '-' || substr('0' || month, -2) || '-' || substr('0' || day, -2) AS `date`
      from foo
     ) t