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?

Answer

For your specific query, you can do:

UPDATE purchaseOrder
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID' and
          not exists (SELECT *
                      FROM itemsOrdered WHERE purchaseOrder_ID = '@purchaseOrdered_ID' AND status = 'PENDING'
                     )

However, I might guess that you are looping at a higher level. To set all such values, try this:

UPDATE purchaseOrder
    SET purchaseOrder_status = 'COMPLETED'
    WHERE not exists (SELECT 1
                      FROM itemsOrdered
                      WHERE itemsOrdered.purchaseOrder_ID = purchaseOrder.purchaseOrdered_ID AND
                            status = 'PENDING'
                      limit 1
                     )