[sql] Insert Data Into Tables Linked by Foreign Key

I am using PostgreSQL.

Customer
==================
Customer_ID | Name

Order
==============================
Order_ID | Customer_ID | Price

To insert an order, here is what I need to do usually,

For example, "John" place "1.34" priced order.

(1) Get Customer_ID from Customer table, where name is "John"
(2) If there are no Customer_ID returned (There is no John), insert "John"
(3) Get Customer_ID from Customer table, where name is "John"
(4) Insert "Customer_ID" and "1.34" into Order table.

There are 4 SQL communication with database involved for this simple operation!!!

Is there any better way, which can be achievable using 1 SQL statement?

Not with a regular statement, no.

What you can do is wrap the functionality in a PL/pgsql function (or another language, but PL/pgsql seems to be the most appropriate for this), and then just call that function. That means it'll still be a single statement to your app.

Use stored procedures.

And even assuming you would want not to use stored procedures - there is at most 3 commands to be run, not 4. Second getting id is useless, as you can do "INSERT INTO ... RETURNING".