Checking Two Tables For Matching Records

[c_pattle]

I was just wondering if anyone knows what the most efficient way of checking to see if two tables contain matching records.

 

So for example I have a table called 'orders' which have a primary key of order_id.

 

Then I have a table called 'order_comments' which also has a field called order_id and I want to run a query that will check which orders don't have entries in order_comments.

 

Thanks for any help

[Barand]

SELECT o.order_id

FROM orders o

LEFT JOIN order_comments c USING (order_id)

WHERE c.order_id IS NULL