Assign row number per ID, where certain values always have fixed numbers - sql

Question

I want to create a query that assigns row numbers per ID in a database table, and certain specific values always get fixed row numbers. For instance, if the value in col2 is A, then the row number should be consistently set to 1. Similarly, if col2 contains the value B, then the row number should always be 2. All other values in col2 should be assigned row numbers in consecutive order starting from 3.
Desired result:
myid col1 col2 row_number
----------------------------------
1 foo A 1
1 bar B 2
1 foobar C 3
1 foobar D 4
2 foobar A 1
2 foob X 3
3 hello B 2
3 hello Z 3
3 hi Y 4
Here is an example which is not working properly.

Answer 1

Sounds like you want to start the row_number with a specific offset, ignoring constant values and assigning them a constant row number.
You can do something a bit ugly like this:
SELECT myid, col1, col2,
case
when col2 = 'A' then 1
when col2 = 'B' then 2
else row_number() over (partition by myid
order by case when col2 = 'A' then 'ZZZ'
when col2 = 'B' then 'ZZZ1'
else col2
end) + 2
end as row_number
FROM newtable
ORDER BY myid, row_number
Result:
MYID COL1 COL2 ROW_NUMBER
1 foo A 1
1 bar B 2
1 foobar C 3
1 foobar D 4
2 foobar A 1
2 foob X 3
3 hello B 2
3 hello Y 3
3 hello Z 4
This start the row number from +2 (Depending on the number of constant values [A,B]), giving each constant value a value that will be sorted last in the row_number window function so the rest will be sorted first.