Override Max Function to allow strings SQL? - sql

Question

Hello what I feel to be a simple question but cannot figure it out. I am trying to find the max number in relation to another column and group it, the issue that comes up is that one of the values is a string.
Name Value
Nate 0
Nate 1
Jeff 2
Nate 2
Nate 'Test'
For the data I actually want 'Test' to be equal to 1. However if I use the MAX() function here I will get:
Name Value
Nate 'Test'
Jeff 2
I can only think that maybe if I read 'Test' as 1 then use the max function (which I am not sure how to do) or possibly overload MAX() to my own definition somehow.
Thank you for any help you can give.

Answer 1

Storing mixed data in a string column is generally a bad idea.
You can convert a specific string to a fixed value with a case expression:
select max(case when value = 'Test' then '1' else value end) from ...
But you are still dealing with strings, so you probably want to convert them to numbers, to prevent '10' sorting before '2' for instance:
select max(to_number(case when value = 'Test' then '1' else value end)) from ...
or
select max(case when value = 'Test' then 1 else to_number(value) end) from ...
Using a CTE for your sample data:
-- CTE for dummy data
with your_table (name, value) as (
select 'Nate', '0' from dual
union all select 'Nate', '1' from dual
union all select 'Jeff', '2' from dual
union all select 'Nate', '2' from dual
union all select 'Nate', 'Test' from dual
)
-- actual query
select name,
max(case when value = 'Test' then 1 else to_number(value) end) as value
from your_table
group by name;
NAME VALUE
---- ----------
Nate 2
Jeff 2
But you have to cover all values that cannot be explicitly or implicitly converted to numbers.
If would be slightly easier if you wanted to ignore non-numeric values, or treat them all as the same fixed value, rather than mapping individual strings to their own numeric values. Then you could write a function that attempts to convert any string and if it gets any exception returns null (or some other fixed value).
From 12cR1 you can even do with with a PL/SQL declaration rather than a permanent standalone or packaged function, if this an occasional thing:
with
function hack_to_number(string varchar2) return number is
begin
return to_number(string);
exception
when others then
return 1;
end;
select name,
max(hack_to_number(value)) as value
from your_table
group by name;
NAME VALUE
---- ----------
Nate 2
Jeff 2
You'd probably be better off going back and redesigning the data model to prevent this kind of issue by using the correct data types.
As #DrYWit pointed out in a comment, from 12cR2 you don't even need to do that, as the to_number() function has this built in, if you call it explicitly:
select name,
max(to_number(value default 1 on conversion error)) as value
from your_table
group by name;

Answer 2

How about this regular expression "trick"?
SQL> with your_table (name, value) as (
2 select 'Nate', '0' from dual
3 union all select 'Nate', '1' from dual
4 union all select 'Jeff', '2' from dual
5 union all select 'Nate', '2' from dual
6 union all select 'Nate', 'Test' from dual
7 )
8 select name, max(to_number(value)) mv
9 from your_table
10 where regexp_like (value, '^\d+$')
11 group by name;
NAME MV
---- ----------
Nate 2
Jeff 2
SQL>