SQL query to get current and last year sales and their difference in oracle

Query using lag for the desired result -

select year_1, total_sale as current_total_sale, 
lag(total_sale) over (order by null) as previous_total_sale,
total_sale - lag(total_sale) over (order by null) difference
from sales;

DB fiddle here

Self join, join the table to itself.

select a.year, a.total_sale as current_total_sale, b.total_sale as prev_total_sale,
b.total_sale - a.total_sale as difference
from have as a
left have as b
on a.year = b.year+1
order by 1;

CREATE TABLE sales (yr,total_sale) AS
SELECT 2015, 23000 FROM DUAL UNION ALL 
SELECT 2016, 25000 FROM DUAL UNION ALL 
SELECT 2017, 34000 FROM DUAL UNION ALL 
SELECT 2018, 32000 FROM DUAL UNION ALL 
SELECT 2019, 33000 FROM DUAL; 

select curr.yr, curr.total_sale, prev.total_sale as previous_total_sale, 
curr.total_sale - prev.total_sale difference
from sales curr
LEFT JOIN sales prev ON curr.yr = prev.yr + 1
ORDER BY curr.yr;

YR  TOTAL_SALE  PREVIOUS_TOTAL_SALE DIFFERENCE
2015    23000    -   - 
2016    25000   23000   2000
2017    34000   25000   9000
2018    32000   34000   -2000
2019    33000   32000   1000