how to union 2 tables but select values based on condition?

Question

there are 2 tables - book, supply. the supply table:

supply_id	title	author	price	amount
1	title1	author1	518.99	2
2	title2	author2	570.20	6
3	title3	author3	540.50	7
4	title4	author4	360.80	3

the book table:

book_id	title	author	price	amount
1	Мастер и Маргарита	Булгаков М.А.	670.99	3
2	Белая гвардия	Булгаков М.А.	540.50	5
3	Идиот	Достоевский Ф.М.	460.00	10
4	Братья Карамазовы	Достоевский Ф.М.	799.01	3
5	Игрок	Достоевский Ф.М.	480.50	10
6	Стихотворения и поэмы	Есенин С.А.	650.00	15

I need to union them but only select those rows that have higher price.

select title, price, amount 
from supply  a1
union all
select title, price, amount 
from book a2
where a1.price > a2.price

Answer 1

Try this:

select title, price, amount 
from supply s
where not exists (select 1 from book b where s.title = b.title and b.price > s.price)

union all

select title, price, amount 
from book b
where not exists (select 1 from supply s where s.title = b.title and s.price > b.price)