SQL - How can I count days by comparing current row to the 1st row?

Question

I have a table as below in the database, how can I write a SQL to show the expected result?

• first_date: the first order_date on the table ORDER BY order_date ASC
• days_to_date: (order_date - first_date) in number of days

My table:

id | order_date | order_ref
---+------------------------
 1 | 2015-03-01 | BC101
 2 | 2015-03-01 | BC102
 3 | 2015-03-02 | BC103
 4 | 2015-03-03 | BC104

Expected result:

id | order_date | first_date | days_to_date
---+------------+------------+-------------
 1 | 2015-03-01 | 2015-03-01 | 0
 2 | 2015-03-01 | 2015-03-01 | 0
 3 | 2015-03-02 | 2015-03-01 | 1
 4 | 2015-03-03 | 2015-03-01 | 2

Other notes:

• I'm using HSQLDB 2.0.0, but prefer solving the problem of getting the first_date displayed on every row in general cases if that's possible

Thanks in advance

Answer 1

Try

select id, order_date, 
      (select min(order_date) from your_table) as first_date,
      datediff('day', (select min(order_date) from your_table), order_date) as days_to_date
from your_table
order by order_date