Auto Generate Unique Number for (Non ID Column) in PostgreSQL Table - database

Question

The PostgreSQL database we have is common multi tenant database.
Question is, need to auto generate a unique number in "customerNumber" column which needs to be in sequential order.
The trick here is, the sequence needs to be unique for each "hotelLocation".
For "hotelLocation"= 1, If we have numbers: 1,2,3 for
"customerNumber"
For "hotelLocation"= 2, We need have numbers: 1,2,3
for "customerNumber"
Following is the sample layout for table,
#Entity
public class CustomerInfo {
#Id
#GeneratedValue(...)
private Long idNumber;
String hotelLocation;
/** Looking for option where, this number needs to
auto generated on SAVE, and need to be in separate sequence
for each hotelLocation **/
private Long customerNumber;
}
So finally here's how output will look like,
+----------+---------------+----------------+
| idNumber | hotelLocation | customerNumber |
+----------+---------------+----------------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 1 |
| 4 | 1 | 3 |
| 5 | 2 | 2 |
+----------+---------------+----------------+
I am ok with generating unique number both via Hibernate based or via Triggers also.
Searching across, i got following,
Hibernate JPA Sequence (non-Id)
But this one would keep generating in sequence without having separate sequence for each "hotelLocation"
Any solution to this will be very helpful. I am sure there are lot of people with multi tenant database looking for similar solution.
Thanks

Answer 1

You can do this easly with postgresql window function row_number().
Don't know your database but it should be something like this:
SELECT idNumber, hotelLocation,
row_number() OVER (PARTITION BY hotelLocation ORDER BY idNumber) AS
customerNumber FROM table
Check more on window functions here: Understanding Window Functions