Question

Given that I have two tables

• Customer (id int, username varchar)
• Order (customer_id int, order_date datetime)

Now I want to insert into Order table based on customer information which is available in Customer table.

There are a couple of ways I can approch this problem.

First – I can query the customer information into a variable and then use it in an INSERT statement.

DECLARE @Customer_ID int
SELECT @Customer_ID = id FROM Customer where username = 'john.smith'
INSERT INTO Orders (customer_id, order_date) VALUES (@Customer_ID, GETDATE())

Second Approach is to use a combination of INSERT and SELECT query.

INSERT INTO Orders (customer_id, order_date)
SELECT id, GETDATE() FROM Customers
WHERE username = 'john.smith'

So my question is that which is a better way to proceed in terms of speed and overhead and why ? I know if we have a lot of information getting queried from Customer table then the second approach is much better.

p.s. I was asked this question in one of the technical interviews.

Answer 1

The second approach is better.

The first approach will fail if the customer is not found. No check is being done to make sure the customer id has been returned.

The second approach will do nothing if the customer is not found.

From an overhead approach why create variables if they are not needed. Set based sql is usually the better approach.