How do I put an ‘if clause’ in an SQL string?

So here’s what I want to do on my MySQL database.

I would like to do:

SELECT *
    FROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status="PENDING"

If that would not return any rows, which is possible through if(dr.HasRows == false), I would now create an UPDATE in the purchaseOrder database:

UPDATE purchaseOrder
    SET purchaseOrder_status="COMPLETED"
    WHERE purchaseOrder_ID = '@purchaseOrder_ID'

How would I be able to make this process a little shorter?