Question

I have some Orders created by Customers stored in postgres db.Some of the Orders may have 'pending' status.I want the user to be able to work on his last pending Order.

order table

id   | order_num | order_date |customer_id | status
-----+-----------+------------+------------+---------
80   | 1234      | 02-01-2000 | 20         | pending
----------------------------------------------------
81   | 2345      | 02-01-2000 |20          | confirmed
-----------------------------------------------------
82   | 3456      | 02-01-2000 |20          | pending
--------------------------------------------------
83   | 3498      | 02-001-2000|  20        | confirmed
----------------------------------------------------

Is there anything wrong with using

select * from orders where customer_id =20 and status='pending' order by id DESC limit 1

The order_date for multiple records may be same.The order number may not always be in ascending order. Is ordering by id safe in this regards?

Answer 1

You should order by the order_date with id as a secondary sort key:

select *
from orders
where customer_id = 20
  and status      = 'pending'
order by order_date desc, id desc
limit 1

You want the most recent order so you should sort to express that intent. You also have to deal with duplicate order_date values so you can throw id in as a secondary sort key to, hopefully, get the most recent one on the most recent order_date. This assumes that id is a serial or bigserial column.

You’re probably safe using order by id desc as long as the order_date is not allowed to change after the record is created. However, if you order by order_date desc, id desc then it will be immediately obvious what your intent is, the people maintaining your code (possibly you) might appreciate it when you saying exactly what you mean.