database - Conditional INSERT INTO statement in postgres

Question

I'm writing a booking procedure for a mock airline booking database and what I really want to do is something like this:

IF EXISTS (SELECT * FROM LeadCustomer 
    WHERE FirstName = 'John' AND Surname = 'Smith') 
THEN
   INSERT INTO LeadCustomer (Firstname, Surname, BillingAddress, email) 
   VALUES ('John', 'Smith', '6 Brewery close,
            Buxton, Norfolk', '[email protected]');

But Postgres doesn't support IF statements without loading the PL/pgSQL extension. I was wondering if there was a way to do some equivalent of this or if there's just going to have to be some user interaction in this step?

Answer 1

That specific command can be done like this:

insert into LeadCustomer (Firstname, Surname, BillingAddress, email)
select 
    'John', 'Smith', 
    '6 Brewery close, Buxton, Norfolk', '[email protected]'
where not exists (
    select 1 from leadcustomer where firstname = 'John' and surname = 'Smith'
);

It will insert the result of the select statement, and the select will only return a row if that customer does not exist.