How to calculate difference between one row with multiple rows sql

Question

I want to calculate the difference between first row with other rows in sql . I have a table named 'Trade' like

date	price
2013-01-01	70
2013-01-02	71
2013-01-03	72
2013-02-01	73
2013-02-02	74
2013-02-03	75

Expected Output:

date	price	first_value
2013-01-01	70	NULL
2013-01-02	71	1
2013-01-03	72	2
2013-02-01	73	NULL
2013-02-02	74	1
2013-02-03	75	2

I used this query:

select date,
    price,
    ABS(first_value(price) over (partition by date_trunc('month', date)) - price)
from trades;

But I get this output:

date	price	first_value
2013-01-01	70	0
2013-01-02	71	1
2013-01-03	72	2
2013-02-01	73	0
2013-02-02	74	1
2013-02-03	75	2

Answer 1

I would do it like this - check if we have the first value in a month, if so, then null :

select 
    date, 
    price, 
    case when month = first_value(month) over (partition by date_trunc('month', date))
        then null
        else ABS(first_value(price) over (partition by date_trunc('month', date)) - price) 
from trades;