How i can to insert after select statement in SQL?

Question

I using django framework and Postgresql develop my project. But I have a problem when many user requests at the same time. It will double inserted data when server is slow. For this reason, I will try to prevent in SQL. I want to know how to SELECT , then INSERT in one command.

Such as

is_created = select created_at,product from payment where created_at=current_date,product_id = '1'

if  is_created == False then 
     insert into table (...) values (....)

Answer 1

postgreSQL supports conditionals you may find this helpful

https://www.postgresql.org/docs/current/functions-conditional.html

example

CASE expression
    WHEN value THEN result
    [WHEN ...]
    [ELSE result]
END